There's more than one way to get the default Python version

[minrk]

Proposed change

Make sure there's only one path to a 'default' python version when Python is unspecified.

Currently CondaBuildPack allows python_version to be an empty string, while PythonBuildPack does not.

Alternative options

Leave it as-is, which means more work to update the default Python version (missed in #1219, suggesting it's a problem).

Who would use this feature?

Maintainers updating the default Python version, users trying to understand how a default Python is chosen

How much effort will adding it take?

Not too much, I think. It should mostly involve ensuring a Python version is set to a default value, rather than allowing it to be an empty string. It should be:

1. remove the environment.lock that lacks a Python version, and
2. whenever python_version would be empty, follow major_pythons.get["3"], as is done here, but not currently in every case
3. assert/assume that python_version is never empty once resolved

this will allow us to assume/ensure that python_version is never empty.

Who can do this work?

Anyone who wants to read through the Python logic of the base conda buildpack and freeze.py.