Fix module init for Python 3.12

[jcrist]

Python 3.12 changed module init slightly (this may actually be a CPython 3.12 bug, it's not 100% clear). Before Python 3.12 the module wouldn't be available via PyState_FindModule until it was fully initialized. Now the module is immediately available, but in an invalid state leading to segfaults. The workaround is to manually call PyState_AddModule to ensure the module is always available.

In the long run we should move to using multiphase module init which should avoid this problem entirely.

[jcrist]

For additional context, the segfault here:

• Only occurs on CPython 3.12
• Only has a faulty memory access at import time. If msgspec successfully imports then you won't be hit by this issue.
• Doesn't segfault if there has been sufficient code run before a msgspec import (I'm guessing this is due to the faulty memory access being in a region that was already allocated and used previously in this case). In particular, this means that the test suite never segfaults due to this issue, which is how it went uncaught until the conda-forge builds started failing (here).

Rather than add a test for this esoteric issue, I've:

• Manually validated that the fix here resolved it
• More finely read the relevant docs which confirm that the code here is as intended
• Read the relevant CPython source to also confirm this behavior

I'll cut a new release immediately after merging this.

[akotlar]

Thanks