String builder, serialization to dict

[glennmatthews]

• Replace all print_detailed APIs with str() methods (not __str__(), as we already have that for these classes, and these methods produce a verbose string rather than a brief description as __str__ does)
• DSyncModel's dict() and json() APIs (inherited from Pydantic) now by default exclude the dsync field as it's not serializable.
• Added dict() APIs on DSync, Diff, and DiffElement, for use in debugging and in serialization
• Unfortunately, having dict() and str() APIs on classes prevents us from using them in type hints - mypy is smart enough to handle this just fine, but pylint started raising used-before-assignment errors, so I've had to replace various dict/str type hints with Dict, Mapping and/or Text types in a number of places.
• Reworked the format of the dict returned by DiffElement's get_attrs_diffs() API so that it produces a dict of the form {src: {key1: value1, key2, value2}, dst: {key1: value1, key2: value2}} instead of the previous {key1: {src: value1, dst: value1}, key2: {src: value2, dst: value2}}, mostly for aesthetic/readability reasons.

Example output of the str() and dict() methods are encoded in the new unit test cases, but here's an example of dict output from a Diff:

{'location': {'ATL': {'_dst': {},
                      '_src': {'parent_location_name': 'USA',
                               'status': 'active'},
                      'device': {'atl-csr1': {'_dst': {},
                                              '_src': {'location_name': 'ATL',
                                                       'model': 'CSR1000v',
                                                       'platform': None,
                                                       'role': 'Router',
                                                       'status': 'active',
                                                       'vendor': 'Cisco'},
                                              'interface': {'GigabitEthernet1': {'_dst': {},
                                                                                 '_src': {'access_vlan': None,
                                                                                          'active': None,
                                                                                          'allowed_vlans': [],
                                                                                          'description': '',
                                                                                          'is_lag': False,
                                                                                          'is_lag_member': False,
                                                                                          'is_virtual': False,
                                                                                          'lag_members': [],
                                                                                          'mode': 'L3_SUB_VLAN',
                                                                                          'mtu': 9000,
                                                                                          'parent': None,
                                                                                          'speed': None,
                                                                                          'switchport_mode': 'NONE'}},
                                                            'GigabitEthernet2': {'_dst': {},
                                                                                 '_src': {'access_vlan': None,
                                                                                          'active': None,
                                                                                          'allowed_vlans': [],
                                                                                          'description': '',
                                                                                          'is_lag': False,
                                                                                          'is_lag_member': False,
                                                                                          'is_virtual': False,
                                                                                          'lag_members': [],
                                                                                          'mode': 'L3_SUB_VLAN',
                                                                                          'mtu': 9000,
                                                                                          'parent': None,
                                                                                          'speed': None,
                                                                                          'switchport_mode': 'NONE'}},
                                                            'GigabitEthernet3': {'_dst': {},
                                                                                 '_src': {'access_vlan': None,
                                                                                          'active': None,
                                                                                          'allowed_vlans': [],
                                                                                          'description': '',
                                                                                          'is_lag': False,
                                                                                          'is_lag_member': False,
                                                                                          'is_virtual': False,
                                                                                          'lag_members': [],
                                                                                          'mode': 'L3_SUB_VLAN',
                                                                                          'mtu': 9000,
                                                                                          'parent': None,
                                                                                          'speed': None,
                                                                                          'switchport_mode': 'NONE'}}}}}},
              'LON': {'_dst': {},
                      '_src': {'parent_location_name': None,
                               'status': 'active'}},
              'NYC': {'_dst': {},
                      '_src': {'parent_location_name': 'USA',
                               'status': 'planned'}},
              'RDU': {'_dst': {},
                      '_src': {'parent_location_name': 'USA',
                               'status': 'active'},
                      'device': {'rdu-n9kv1': {'_dst': {},
                                               '_src': {'location_name': 'RDU',
                                                        'model': 'Nexus 9000v',
                                                        'platform': None,
                                                        'role': 'Switch',
                                                        'status': 'active',
                                                        'vendor': 'Cisco'}}}},
              'USA': {'_dst': {},
                      '_src': {'parent_location_name': None, 'status': None}}}}

[glennmatthews]

Fixes #22.

[dgarros]

Looks good to me, thanks