gh-101758: Add a Test For Single-Phase Init Module Variants

[ericsnowcurrently]

The new test exercises the most important variants for single-phase init extension modules. We also add some explanation about those variants to import.c.

• Issue: Isolate PyModuleDef to Each Interpreter for Extension/Builtin Modules #101758

[gpshead]

!buildbot Fedora Stable Refleaks

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @gpshead for commit b7bf588 🤖

The command will test the builders whose names match following regular expression: Fedora Stable Refleaks

The builders matched are:

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @gpshead for commit b7bf588 🤖

If you want to schedule another build, you need to add the 🔨 test-with-refleak-buildbots label again.