Add Python 3.8; drop Python <3.6

[crusaderky]

• Closes Python 3.8 DeprecationWarnings; drop support for legacy Python #133
• Added support for Python 3.8
• Dropped support for Python <3.6
• Deprecated 'loop' parameter in Connection.create and Pool.create
• Fixed all DeprecationWarnings
• Fix missing object asyncio.streams.IncompleteReadError
• Fixed all Sphinx build errors and warnings
• Intersphinx: all classes that can be imported from the top-level module must now be
  referenced from the top-level module in the docstrings too.
  E.g. :class:`asyncio_redis.exceptions.Error` has changed to
  :class:`asyncio_redis.Error` .

[crusaderky]

@jonathanslenders ready for review

[crusaderky]

Hi @jonathanslenders, any feedback on this?

[jonathanslenders]

Hi @crusaderky,

Thank you so much for this! It looks great.
If you think it's ready and well tested, I can merge it.

[crusaderky]

@jonathanslenders I tested it in my Python 3.8 project, which however uses only a tiny subset of the methods, and it seems to work fine. For everything else, we have to rely on the unit tests. I'd say merge...

[jonathanslenders]

Merged! Thanks a lot for this!

[jbasko]

This was never released, was it? Is the idea to use a version pointing to git commit?