Documentation for Windows users can be simpler now that we only support Python >= 3.5

[Ramiro Morales]

Python installations on WIndows now

• Ship the PEP397 py launcher which reduces the need to either having to add python.exe directory to PATH or having to always aspecify its full path. OIt also cand detect your Python's python.exe even when its invoked from inside a virtualenv.
• Have a better "Execute manage.py or runtest.py by simply running them" story when compared to previous Python versions, thanks partially to the above mentioned launcher.

This would allow us to reduce special casing of Windows when describing command line interactions. In particular IMHO the suggestion to use the Git for Windows Bash shell can be dropped from the contributing tutorial as it doesn't actually make things easier.