With Python 3.8, unpickling pynac objects gives a deprecation warning

DeprecationWarning: PY_SSIZE_T_CLEAN will be required for '#' formats

These are fixed in this ticket by defining PY_SSIZE_T_CLEAN where appropriate

Upstream pull request: pynac/pynac#354

Upstream: Reported upstream. No feedback yet.

CC: @jhpalmieri @mkoeppe

Component: packages: standard

Author: Antonio Rojas

Branch/Commit: ecc6f45

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/30225