devel/py-virtualenvwrapper: Allow Python 3 builds

virtualenvwrapper supports Python 3, remove unnecessary limitation on 2.7
accordingly.

A dependency (sphinxcontrib-bitbucket) Python 3 build/run error was
fixed in ports r508699, which is required by virtualenvwrapper for its
DOCS option/build.

While I'm here:

• Level up ports compliance (NO_ARCH, USES/USE order)
• Remove unecessary DOCS_VARS DOCSDIR override (concurrent does this)
• Patch virtualenvwrapper command invocations (virtualenv, virtualenv-clone) to use Python version-specific filenames

Approved by: portmgr (blanket(s): ports compliance, python compliance)
MFH: 2019Q3