Tests for PY3-only features / keywords

[sundarnagarajan]

Just adding as-is to existing script will break PY2 testing - since PY2 interpreter will not load the script with unknown keywords even if protected with "if PY2"

Options:

• Separate top-level test script
• Separate PY3-only test module selectively loaded within one or more test cases
• Using "eval" or the like

Python3-only test cases:

• Test case wrapping async context manager
  • aenter, aexit
  • See link
• Test case wrapping coroutine
  • 'send', 'throw', close'