[Testing] Migrate completely to `pytest`

[abhinavsingh]

Is your feature request related to a problem? Please describe.
Currently, a lot of testing code from unittest is mixed with pytest eco-system. We should migrate to pytest recommendations and give up unittest style tests.

Describe the solution you'd like

• This includes abstracting out various fixtures so that they can be easily re-used across tests
• Currently there is a lot of redundancy in tests which can be addressed using pytest.parameterize
• fixtures will allow new contributors to easily understand and write new tests, instead of mocking 6-layer deep.

Ref discussions

• Async get_events, handle_event, handle_readables, handle_writables #769 (comment)
• Async get_events, handle_event, handle_readables, handle_writables #769 (comment)
• https://github.com/abhinavsingh/proxy.py/pull/769/files#r753837986
• https://github.com/abhinavsingh/proxy.py/pull/769/files#r753837247

[webknjaz]

Pylint has a feature to forbid imports of one module and recommend another. You could add the following to .pylintrc:

[IMPORTS]
preferred-modules =
    unittest:pytest,