gh-121306: allow a mapping as __dict__ of object

[blhsing]

This PR allows a mapping as the __dict__ attribute of an object. This was originally meant to allow a mapping to be passed as the globals argument to exec and eval, but for consistency the global namespace of a module should be allowed to be a mapping as well, and by extension the __dict__ attribute of any object.

Full backwards compatibility is maintained by adding a PyDict_Check-guarded fast path to existing PyDict_* function calls with a fallback to their PyMapping_* equivalent API calls. A unified dict/mapping API in the form of helper macros is introduced to avoid potential code clutter from adding such a conditional statement to every PyDict_* call.

Note that some PyDict_* functions do not have direct PyMapping_* equivalents so calls to those functions have to be switched to similar functions that do have PyMapping_* equivalents. These include:

• PyDict_Pop(dict, key, NULL) is replaced with PyDict_DelItem(dict, key); the former does not raise KeyError while the latter does, so the caller has to handle error differently.
• PyDict_GetItemWithError is replaced with PyDict_GetItemRef; the former returns a borrowed reference while the latter creates a new ref so the caller has to account for the new ref accordingly.

Tests that expected SystemError or TypeError from a non-dict global namspace have been revised to reflect the new capability.

• Issue: missing global names inside a class body (calling exec with a ChainMap) #121306