gh-111178: fix UBSan failures in `Objects/exceptions.c` (GH-128154)

Fix UBSan failures for `PyBaseExceptionObject`, `PyStopIterationObject`, `PySystemExitObject`, `PyImportErrorObject`, `PyOSErrorObject`,  `PyNameErrorObject`,  `PyAttributeErrorObject`, `PySyntaxErrorObject`,  `KeyError`,  `UnicodeError*`, `PyBaseExceptionGroupObject`

Remove redundant casts

Align the naming convention `Py[...]_CAST(op)` where only an assert-only type check is performed.