HTTP Client

[apetenchea]

This PR lays the foundation for the asynchronous HTTP Client

The following classes are introduced:

• HTTPClient - abstract base class
• AioHTTPClient - client implementation, based on aiohttp
• DefaultHTTPClient - alias for AioHTTPClient
• Request - request skeleton (might be subject to change later)
• Response - response skeleton (might be subject to change later)
• Method - using an enum instead of a string because we have more control over what values we'll put in it, and because most editors can make sense of it and offer better type hints

Testing
Basic unit tests have been introduced, see test_http.py. Test configuration will be in conftest.py. ATM I'm testing it locally, it's a bit early for a full CI. I'll make sure we get that as soon as we have a few extra tests. For now, just do pytest -s.

Documentation
We've got docstrings so far, and the sphinx build is working: python -m sphinx docs docs/_build. The cool thing is that we can now point to the documentation of other projects, for example whenever a parameter has type aiohttp.BasicAuth, the docs are actually going to point to that type, and the project's documentation url is derived automatically using the intersphinx_mapping.
I wanted to add support for numpy docstrings, because of their concise format, but that would be in a later PR, see this example for details.

Version adjustments
Some of the pre-commit tools were bumped to a more recent version (we should do that at some point also at python-arango.

Note: I have created a page for development ideas

[aMahanna]

LGTM