Description

msgspec currently uses the old style single-phase init for module import. We should transition to use the newer multi-phase init process. Besides code quality, this is one of the steps required for us to make use of the new per-interpreter GIL subinterpreters feature in CPython 3.12.

Some relevant resources:

• https://peps.python.org/pep-0489/
• https://docs.python.org/3.12/howto/isolating-extensions.html
• https://peps.python.org/pep-0573/