devel/py-mock: Take MAINTAINER'ship, level up ports compliance

• Use upstream (setup.cfg) summary as COMMENT
• Fix dependencies (six is not a build time dependency)
• Use GitHub (tests not packaged in PyPI sdist)
• Standardize pytest invocation (show skip reasons, one line per test)
• Use upstream (setup.cfg) description as pkg-descr
• Use upstream (setup.cfg) home-page as pkg-descr WWW URL

PR: 241780
Approved by: portmgr (unmaintained port. blanket(s): ports compliance, bugfix(es))
MFH: No (3.x might be API incompatible for consumers, unclear)