gh-122581: Use parser mutex in default build for subinterpreters

[colesbury]

The data race also affected subinterpreters in the default build.

• Issue: TSan reported race condition in _PyPegen_is_memoized in test_importlib #122581

[colesbury]

This was caught by running ./python -m test test_datetime -m test_concurrent_initialization_subinterpreter -v with TSan, but that test still can't be enabled because there are other unfixed data races.

[ZeroIntensity]

Ouch, I wrote that test as part of #136583. What other races are you seeing?

[colesbury]

@ZeroIntensity - Mostly things from #129824

[ZeroIntensity]

There's not much else to discuss here, LGTM.