Upgrade existing code to Python 3.7

[geotom]

Tasks

• See where we can use Data Classes (https://docs.python.org/3.7/whatsnew/3.7.html#whatsnew37-pep557)
• Typing now got additional support y PEP 560 (https://docs.python.org/3.7/whatsnew/3.7.html#whatsnew37-pep560)
• Introduction of "Postponed Evaluation of Annotations" should render the creation of TypeVars unnecessary (https://docs.python.org/3.7/whatsnew/3.7.html#whatsnew37-pep563). See for instance server/client base classes *Argparse has a new parse_intermixed_args() method (https://docs.python.org/3.7/library/argparse.html#argparse.ArgumentParser.parse_intermixed_args)
• Adopt to changes and recommendations in asyncio (https://docs.python.org/3.7/whatsnew/3.7.html#whatsnew37-asyncio)
  • Change for instance: ensure_future -> create_task?
  • The new StreamWriter.wait_closed() coroutine method allows waiting until the stream writer is closed.
  • The asyncio.Task.current_task() and asyncio.Task.all_tasks() methods have been deprecated.
  • ProcessPoolExecutor and ThreadPoolExecutor now support the new initializer and initargs constructor arguments.
  • The ProcessPoolExecutor can now take the multiprocessing context via the new mp_context argument.
• Enum: The Enum learned the new ignore class property, which allows listing the names of properties which should not become enum members.
• os.scandir() gained support for file descriptors
• Sockets
  • The new socket.close() function closes the passed socket file descriptor. This function should be used instead of os.close() for better compatibility across platforms
  • Sockets now auto-detect family, type and protocol from file descriptor by default
• SSL: The ssl module now uses OpenSSL’s builtin API instead of match_hostname() to check a host name or an IP address. Values are validated during TLS handshake. Any certificate validation error including failing the host name check now raises SSLCertVerificationError and aborts the handshake with a proper TLS Alert message. The new exception contains additional information. Host name validation can be customized with SSLContext.host_flags

Todo

• Try to implement first missing changes of #1
• Check if tests work afterwards

[geotom]

• Asyncio:
  • Lock, Condition, Semaphore, and BoundedSemaphore objects can be used in async with statements.

[geotom]

More asyncio changes in 3.7 here! https://tryexceptpass.org/article/asyncio-in-37/

Of interest are the conextvars and asyncio.run()

Regarding asyncio.createask, it is better to revert to the old ensure_future for a specific loop, otherwise create_task takes a default one and not the one of the transport!

Also see Async Context Managers: Another quality-of-life improvement. We now have the asynccontextmanager() decorator for producing async context managers without the need for a class that implements aenter() or aexit(). This behaves exactly like the contextmanager() decorator that we use today for synchronous code. They also added a new AbstractAsyncContextManager and AsyncExitStack to complement their synchronous cousins.

Check the server state (aka isRunning?) We can now control when an asyncio.Server begins serving during creation with the start_serving keyword and the Server.start_serving() function. Plus we can also have Server.is_serving() to determine its state.

Loop methods which accept socket paths now support passing path-like objects.

[geotom]

Dataclasses can be combined with jsonschema via this module: https://pypi.org/project/dataclasses-jsonschema/