bpo-21016: pydoc and trace use sysconfig (GH-18476)

bpo-21016, bpo-1294959: The pydoc and trace modules now use the
sysconfig module to get the path to the Python standard library, to
support uncommon installation path like /usr/lib64/python3.9/ on
Fedora.

Co-Authored-By: Jan Matějek <jmatejek@suse.com>