Python 3.7 Compatibility: Py_UnicodeAsUTF8AndSize() returns a "const char*"

instead of a "char*".

(See https://docs.python.org/3/whatsnew/3.7.html#c-api-changes)

There are additional changes needed for Python 3.7 compatibility,
this change just pulls out one of them
(and subsumes a related attempt in #21202 and #20766)

Helps with #20517

PiperOrigin-RevId: 207008013