PiP upgrade no longer works for Python 2

[haxorof]

Because PiP has dropped support for Python 2 in version 21 it is no longer possible to just do a simple pip install -U pip on a system with Python 2 like CentOS 7.

This role has to now ensure that PiP upgrade only allow 21 or later for Python 3 and below that for Python 2.

https://pip.pypa.io/en/latest/development/release-process/#python-2-support

pip 20.3 was the last version of pip that supported Python 2. Bugs reported with pip which only occur on Python 2.7 will likely be closed as “won’t fix” issues by pip’s maintainers.