Add support for python_platform_implementation environment marker

Extract a a method to encapsulate behavior and documentation. Rewrite for loop as legacy-style dictionary comprehension.

Fix "dictionary changed size during iteration"

env is being modified at the same time as being iterated which leads to RuntimeError: dictionary changed size during iteration.

Dropping support for Python 3.2 and earlier Python 3 versions. Currently only log a warning, but future versions will fail.

This patch adds support for the 'python_platform_implementation'
environment marker as defined by PEP-0426:
https://www.python.org/dev/peps/pep-0426/#environment-markers