devel/py-virtualenv: Update to 20.16.7

• Update (BUILD|RUN|TEST)_DEPENDS list
• Update patch for docs/conf.pyp file
• Use pytest for testing
• Set the environment variable SETUPTOOLS_SCM_PRETEND_VERSION to fix the exception:
  
  LookupError: setuptools-scm was unable to detect version

• Disable QA_depends_blacklist when executing the test target, because virtualenv also requires python2 executable for some tests
• Set the test environment variable PYTHONPATH, so pytest can find the virtualenv library
• Force setuptools to create a temporary egg_info in build/lib during the build stage, so that sphinx can then create docs. Exception occurred:
  
  File "/usr/local/poudriere/ports/default/devel/py-virtualenv/work-py39/virtualenv-20.16.7/build/lib/

virtualenv/run/plugin/discovery.py", line 20, in get_discover

default=next(iter(choices)),

StopIteration

(raised only when virtualenv is not already installed in the system)

• Add devel/py-importlib-metadata to RUN_DEPENDS when using python3.7
• Reorder variables in Makefile

Port tested with all versions of Python currently supported.

Changelog: https://github.com/pypa/virtualenv/releases/tag/20.16.7