Python 3.5.3 compatibility

[N-Coder]

  File "/home/finksim/src/studip-fuse/studip_fuse/studipfs/api/aiointerface.py", line 4, in <module>
    from typing import Any, AsyncContextManager, AsyncGenerator, Callable, Coroutine, Dict, Generic, NamedTuple, Optional, TypeVar, Union
ImportError: cannot import name 'AsyncContextManager'

  File "/home/finksim/src/studip-fuse/studip_fuse/studipfs/api/aiointerface.py", line 4, in <module>
    from typing import Any, AsyncGenerator, Callable, Coroutine, Dict, Generic, NamedTuple, Optional, TypeVar, Union
ImportError: cannot import name 'AsyncGenerator'

  File "/home/finksim/src/studip-fuse/studip_fuse/launcher/aioimpl/asyncio/aiohttp_client.py", line 115, in DownloadState
    EMPTY = enum.auto()
AttributeError: module 'enum' has no attribute 'auto'

  File "/home/finksim/src/studip-fuse/studip_fuse/studipfs/api/session.py", line 6, in <module>
    from typing import AsyncGenerator, List, Mapping, Tuple
ImportError: cannot import name 'AsyncGenerator'

typing.AsyncGenerator is new in version 3.5.4
typing.AsyncContextManager is new in version 3.6
enum.auto is new in version 3.6

[N-Coder]

should also check whether we could use an old aiohttp version under Ubuntu 16.04

[N-Coder]

typing.AsyncContextManager is backported by typing_extensions, typing.AsyncGenerator is only exported by typing_extensions if related classes are also available, which is not the case on 3.5.3. It is available trough trio-typing (which also requires trio and mypy).

enum.auto() can be simulated by using the functional API, using the aenum package or simply defining the int values explicitly.

[ClusterJan]

Testing the new "Python3.5" branch results in a new error:

Uncaught exception from FUSE operation readdir, returning errno.EINVAL: argument of type 'NotImplementedType' is not iterable
Traceback (most recent call last):
[...]
  File "/home/aschwege/.local/lib/python3.5/site-packages/cached_property.py", line 35, in __get__
    value = obj.__dict__[self.func.__name__] = self.func(obj)
  File "/home/aschwege/.local/lib/python3.5/site-packages/studip_fuse/avfs/virtual_path.py", line 231, in content_options
    if field_name not in self.format_token_generators:
TypeError: argument of type 'NotImplementedType' is not iterable

[N-Coder]

__init_subclass__ used in virtual_path.py:215 to initialize the format_token_generators was added in 3.6

[N-Coder]

should be fixed by d644f57 and 8558a4f