Drop support for Python 2.7, 3.5

As mentioned at the 36th Tango Community Meeting in June 2022, these old versions of Python will be dropped from PyTango 9.4.0.

https://github.com/ajoubertza/pytango-status-updates/blob/2022-06/slideshow.md#upcoming-release---940

Changes include:

• Update python_requires and classifiers in setup.py.
• Remove Gitlab and AppVeyor CI for old platforms.
• Remove usage of exec() in tests that couldn't define async def functions - search for ASYNC_AWAIT_AVAILABLE.
• Check docs for mention of old Python versions.