Selecting a specific Minor version of Python.

[ablakey]

The documentation explains that we can pick which major version of Python is used via: PYTHON_VERSION_MAJOR 3 # Default 2

However, as an implementation detail, we can actually pass minor versions too. Eg:
PYTHON_VERSION_MAJOR 3.7 works.

I'm thinking about how to formalize this capability without breaking backwards compatibility. The short term answer is to document this quirk. I think maybe more complete solution is to replace PYTHON_VERSION_MAJOR with PYTHON_VERSION while retaining support for the former.

@paulbovbel