gh-134009: Expose `PyMutex_IsLocked` in the public C API (gh-134365)

The `PyMutex_IsLocked()` function is useful in assertions for verifying
that code maintains certain locking invariants.

• Doc/c-api/init.rst[diff]
• Doc/whatsnew/3.15.rst[diff]
• Include/cpython/lock.h[diff]
• Include/internal/pycore_lock.h[diff]
• Misc/NEWS.d/next/C_API/2025-05-20-17-13-51.gh-issue-134009.CpCmry.rst[Added - diff]
• Python/lock.c[diff]