We should support the .python-version file as a means for specifying what Python version an app is using.

This file format (used by pyenv and others) now also supports specifying Python versions in the X.Y format (and not just X.Y.Z), making it a strong contender over the Heroku-specific runtime.txt file.

Officially supported syntax:

• 3.12.6
• 3.12
• comments (lines starting with #)
• blank lines
• pyenv supports specifying multiple versions, however, this buildpack can't, so we'll either want to only use the first version, or error if there are multiple versions specified

Unofficially supported syntax:

• python-3.12.6
• python-3.12 (though this currently has a bug: The auto-resolution of latest versions feature doesn't work with python- prefixes (eg python-3.12) pyenv/pyenv#3054)

...though the python- forms are undocumented and likely going to be deprecated soon (pyenv/pyenv#3054 (comment)), so I'm leaning towards not supporting them.

For more background, see:

• Python Version via .python-version heroku-buildpack-python#932
• Support flexible patch version syntax to allow for automatic Python updates heroku-buildpack-python#913 (comment)
• Internal tracking epic