More details:
- Fix a race while enabling the GIL by checking if the GIL was enabled between
  a no-op call to `_PyEval_AcquireLock()` and the thread attaching, and trying
  again if it was.
- Enable the GIL before running a module init function, since we can't know if
  it's a single-phase init module that doesn't support free-threading. Look at
  the state of the module after initialization to determine if it's safe to
  disable the GIL again.
- Add `PyModule_SetGIL()`, which can be used by single-phase init modules to
  declare that they support running without the GIL.
- Change `gil->enabled` from a simple on/off switch to a count of active
  requests to enable the GIL. This allows us to support multiple interleaved
  imports that each independently track whether the GIL should remain
  enabled. See the big comment in `pycore_ceval.h` for more details.

Also fix the module format string for normal builds, since it only contains
pointers (no `size_t`).

…IL()