Use latest patch version

[CaseyFaist]

This will allow users to specify a python major and minor branch to always use the latest patch available.

python-X.X.* is the acceptable format, where X.X are the major and minor versions.

Available versions for this tooling:

python-3.8.*
python-3.7.*
python-3.6.*
python-2.7.*

[CaseyFaist]

Hopefully will get to come back around to this next week, I'm excited about this feature 🎉

[edmorley]

I think this would cause false positives - $PYTHON_VERSION == *"3.8"* will match against versions like *.3.8 or 2.3.8*

[edmorley]

This is definitely something I'd like the buildpack to support (and as soon as possible!).

I've added some thoughts as to implementation approaches to #913 - closing this PR out since I'll open a new one once I start working on this :-)