fuzz: fix Makefile default PYTHON_CONFIG_FLAGS to be modern
This is actually what we already do on oss-fuzz, so it's more correct as tests
go.
Differential Revision: https://phab.mercurial-scm.org/D9790
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.