community/py3-python-gssapi: upgrade to 1.6.12, add tests

This MR includes:

• Tests are finally enabled (as py3-k5test now graduated to community tree)
• Upgrade to 1.6.12 (changelog below)

https://github.com/pythongssapi/python-gssapi/releases

1.6.12

• Fix GSSAPI detection for macOS Big Sur (235, 236)
• Clarify how to disable detection (234)

1.6.11

• Fix build on FreeBSD (232, 228)
• On FreeBSD, prefer GSSAPI from ports if available (232)
• Support python >= 3.8 in test suite by dropping dependency on should_be (215, 229)
• Fix malformed S4U tests (220, 229)

1.6.10

• Provide python 3.9 Windows wheels (226, 227)
• Fix shlex.split on Windows (223)
• Removed six package and other older py cleanup values (221, 222)