Docs: Replace hardcoded SOURCE_URI with patchlevel check

[hugovk]

During yesterday's 3.15.0b2 release, the reminder to update SOURCE_URI triggered.

SOURCE_URI is used for linking stdlib pages such as:

• https://docs.python.org/3/library/random.html -> https://github.com/python/cpython/tree/3.14/Lib/random.py
• https://docs.python.org/3.15/library/random.html -> https://github.com/python/cpython/blob/3.15/Lib/random.py

But this reminder was meant to happen during b1, when the 3.15 branch is created. It happened during b1 for 3.14.

See #150783 (comment).

The cause of this was this fix: python/release-tools#379.

But rather than updating release-tools to trigger the reminder during b1 (and it's a disruptive reminder), let's remove the hardcoding in SOURCE_URI altogether, and replace it with a check for the release level:

• For alpha (and that includes main as it is now, pre-a1): link to main.
• Otherwise: link to the relevant 3.x.

And we do this directly where it's used in conf.py instead of in pyspecfic.py and importing from there.

Also remove a bunch of unused imports pyspecfic.py.

[StanFromIreland]

We already calculate the version above, can we re-use that?

cpython/Doc/conf.py

Lines 78 to 81 in 6453065

	# We look for the Include/patchlevel.h file in the current Python source tree
	# and replace the values accordingly.
	# See Doc/tools/extensions/patchlevel.py
	version, release = import_module('patchlevel').get_version_info()

[hugovk]

They're strings like "3.16" and "3.16.0a0". I suppose we could do "a" in release, but it's cleaner to check releaselevel.

But we can clean up these imports: no need for importlib.import_module because we've already done sys.path.append(os.path.abspath('tools/extensions')).

[StanFromIreland]

I tested it in the preview by going to this section and clicking the last link and I can confirm it works :-)

[StanFromIreland]

LGTM

[miss-islington-app]

Thanks @hugovk for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14, 3.15.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @hugovk, I could not cleanly backport this to 3.15 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker b643826c1164274e35f55d2236aa47085e2a5efc 3.15

[miss-islington-app]

Sorry, @hugovk, I could not cleanly backport this to 3.14 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker b643826c1164274e35f55d2236aa47085e2a5efc 3.14

[miss-islington-app]

Sorry, @hugovk, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker b643826c1164274e35f55d2236aa47085e2a5efc 3.13

[bedevere-app]

GH-150855 is a backport of this pull request to the 3.15 branch.

[bedevere-app]

GH-150856 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-150857 is a backport of this pull request to the 3.13 branch.