Add Api.create_token() (for NetBox 3.0+)

[markkuleinio]

NetBox 3.0.0 release notes: https://netbox.readthedocs.io/en/stable/release-notes/version-3.0/#rest-api-token-provisioning-5264

Example with this PR:

>>> import pynetbox
>>> netbox = pynetbox.api("https://netbox-server")
>>> token = netbox.create_token("admin", "netboxpassword")
>>> netbox.token
'96d02e13e3f1fdcd8b4c089094c0191dcb045bef'
>>> from pprint import pprint
>>> pprint(dict(token))
{'created': '2021-11-27T11:26:49.360185+02:00',
 'description': '',
 'display': '045bef (admin)',
 'expires': None,
 'id': 2,
 'key': '96d02e13e3f1fdcd8b4c089094c0191dcb045bef',
 'url': 'https://netbox-server/api/users/tokens/2/',
 'user': {'display': 'admin',
          'id': 1,
          'url': 'https://netbox-server/api/users/users/1/',
          'username': 'admin'},
 'write_enabled': True}
>>>

I also tested implementing this as Endpoint.provision() (just like it is in NetBox API: /api/users/token/provision/), but then an extra if self.name == "token": self.api.token = resp["key"] would be needed to set the token, and I just think this is a clear way to create the token when needed.