run-tests: drop support for --with-python3 and $PYTHON3
This backs out
cdbc25306696. We can run many tests on python3 without
this hack.
test-check-py3-compat: don't run py3 part if not build against python3
This test depends on C extensions built for python3, which aren't available
if run-tests.py runs on python2 and no -l/--local is specified.