fuzz: try and ensure fuzzer tests run against the right python-config
Also only under python 3.
Differential Revision: https://phab.mercurial-scm.org/D9752
#require black test-repo $ cd $RUNTESTDIR/.. $ black --config=black.toml --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`