Add --force-python option as shorthand for --python and --extra-python

[cjolowicz]

This PR adds an option --force-python / --force-pythons which functions as a shorthand for when the same interpreter spec should be passed to both --python and --extra-python. This has the effect of running sessions on the specified interpreter instead of those listed for each session.

In other words, these two are equivalent:

• nox --python=X.Y --extra-python=X.Y
• nox --force-python=X.Y

By comparison, --extra-python on its own adds an interpreter to those listed for a session. The sessions will be run on the original Python versions as well as the extra version.

This came up a few times in the past:

• feat: support users specifying an undeclared parametrization of python #361 (comment)
• --extra-python does not replace Python version by full path on Windows #412 (comment)
• use base python installation (not virtual environment's Python) during python interpreter resolution #233 (comment)

[theacodes]

This looks great! Thank you. :)