GeezerVison

Description:
This introduces a python-versions output, which is a comma-separated string of all installed Python versions and implementations. This is useful for cache-busting when multiple Python versions are installed, as described in #606.

For example, it is now possible to cache a .tox/ test directory and invalidate the cache when any of the Python versions change. Caching the .tox/ directory offers a significant speedup to test setup times, but if a non-default Python's version changes (say, if 3.11.1 is the default but 3.10.x increments to 3.10.y), it is currently very difficult to invalidate the cache. This new output variable allows for trivial cache invalidation in this situation.

To demonstrate the new output variable, I created a workflow with a single setup-python step that installs multiple CPython and PyPy versions, then used echo to show the value of python-versions.

The value displayed is:

3.10.9,3.11.1,3.12.0-alpha.5,3.7.15,3.8.16,3.8.16-pypy7.3.11,3.9.16,3.9.16-pypy7.3.11

which can be used directly in a cache key, or written to a file and included in a call to hashFiles(). The versions are sorted to avoid sensitivity to the order of Python versions specific in the workflow, but are sorted merely as strings for this purpose, not as semantic versions. Therefore a user can create a workflow that installs Python versions in any order they want, but the python-versions output string will be stable.

NOTE: This work depends on #610 to remove trailing newlines in PyPy versions.

Related issue:
Closes #606

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.