gh-100227: Make the Global Interned Dict Safe for Isolated Interpreters

[ericsnowcurrently]

This is an alternative to gh-102339, where we keep the interned dict global instead of making it per-interpreter.

• Issue: Improve Interpreter Isolation #100227

[ericsnowcurrently]

We make the interned strings immortal separately in gh-102887.

[ericsnowcurrently]

This is superceded by gh-102925, except for the part where we add a granular lock, which would need to be taken care of for a per-interpreter GIL.