Added Python 3.11 support

[qstokkink]

Fixes #1110

This PR:

• Adds an __all__ definition for base.py to avoid exporting newly added module-level variables.
• Adds TestBase support for Python 3.11 using unittest.IsolatedAsyncioTestCase.
• Fixes RandomChurn using random.sample on a set (only list is supported as of Python 3.11).

See original issue for implementation substantiation. Other notes:

• TestBase now inherits from unittest.IsolatedAsyncioTestCase for all Python versions >= 3.8 by default. However, asynctest is still supported up to Python 3.10 and TestBase subclasses can switch to asynctest by setting __asynctest_compatibility_mode__ to True.
• loop has a class scope in asynctest, an instance scope in unittest, and it needs to be retrieved upon invocation (a property) for Python 3.11.
• _callSetUp and _callTearDown are overwritten to support the old TestBase behavior of allowing both async def and def setup and teardowns, including its own. For (only) the teardown of TestBase, the asyncTearDown is still explicitly needed.

[qstokkink]

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