[3.6] bpo-35011: Restore use of pyexpatns.h in libexpat (GH-9939) (GH-9941)

[3.6] bpo-35011: Restore use of pyexpatns.h in libexpat (GH-9939) (GH-9941)

Restores the use of pyexpatns.h to isolate our embedded copy of the expat C
library so that its symbols do not conflict at link or dynamic loading time
with an embedding application or other extension modules with their own
version of libexpat.

https://github.com/python/cpython/commit/5dc3f23b5fb0b510926012cb3732dae63cddea60GH-diff-3afaf7274c90ce1b7405f75ad825f545 inadvertently removed it when upgrading expat.
(cherry picked from commit 9d4712bc8f26bf1d7e626b53ab092fe030bcd68d)

Co-authored-by: Gregory P. Smith <greg@krypto.org>
https://bugs.python.org/issue35011