Switch from get-pip.py to ensurepip

[edmorley]

Since:

• All versions of Python that are actively built by this repo now include the ensurepip module.
• The policy of these images is now to use the same pip version as the one bundled with ensurepip (rather than always upgrading as pip releases occur) to avoid breaking changes, and for parity with the venv module.
• As such, we might as well actually use ensurepip to install pip (since it installs the exact pip version we want) rather than manually doing the same using get-pip.py.
• Doing so simplifies the build scripts and removes an external dependency, tightening the supply chain.

Now that the pip/setuptools versions track (or mostly track, in the case of setuptools) the ensurepip versions, the concerns over frequent invalidation of the Python layer no longer apply, and so the pip/setuptools install can now be part of the Python layer, reducing layer count by one.

This change is a no-op in terms of pip/setuptools/wheel versions, since the pip versions being used already exactly matched the ensurepip version of pip.

Closes #951.

[tianon]

Excellent, thank you! Just a few really minor stylistic notes, but overall this looks really good. ❤️

[edmorley]

Thank you for the feedback! I've made the those changes - I wouldn't normally force push for review changes, but since this repo uses merge commits rather than squash and merge I wanted to avoid the additional commits ending up on master. However, the diff from before can still be seen here:
https://github.com/docker-library/python/compare/7eab18fc115a913534bb0bc5c40bf45c26b61b05..0a4fcf7342f030937b1aad2848d07c0c995bf713

Happy to make further changes if you spot anything else :-)

(And also no rush to merge this; we can wait until the other changes are released first etc)

[tianon]

Last thing I see -- my mental model here is that the closing ; on a long multi-line is similar to the closing ) or } so I outdent it to make it clear this is the end of the extra long command+args. 👍

Thanks for your work on this! ❤️

[edmorley]

I was going to rebase this on master after last night's Python version releases (which have caused conflicts with this PR), however, for some reason the Python 3.12.6 update hasn't been pushed yet by the automation? Did that job fail? (I don't know where those jobs run)
https://github.com/docker-library/python/commits/master/

[edmorley]

I've rebased on master now that the bump for #960 has landed.

[tianon]

Also, to be really explicit, thank you so much for bringing up the original thread, being so thoughtful and thorough, seeing it all the way through including the extra cleanup of get-pip vs ensurepip here, and generally helping respond to the folks that were broken by the whole transition -- it's really appreciated. ❤️

[edmorley]

That's very kind of you to say, it's been no trouble! :-)