C API: Remove private functions from abstract.h

[vstinner]

Over the years, we accumulated many private functions as part of the public C API in abstract.h header file. I propose to remove them: move them to the internal C API.

Linked PRs

• gh-106084: Remove old PyObject call aliases #106085
• gh-106084: Remove _PySequence_BytesToCharpArray() function #106088
• gh-106084: Remove _PyObject_HasLen() function #106103
• gh-106084: Remove _PyObject_RealIsInstance() function #106106
• gh-106084: Remove _PyObject_CallMethod() function #106159