tests: conditionalize test-run-tests.t for Python 3.8
It looks like Python 3.8 changed the pretty XML formatting slightly
to strip some whitespace. Let's teach our tests about that.
Differential Revision: https://phab.mercurial-scm.org/D6988
hgweb: use importlib.reload() if available
reload() was nuked in Python 3. We need to use importlib.reload()
instead.
But pyflakes isn't smart enough to detect our conditional usage, so
we allow this error.
Differential Revision: https://phab.mercurial-scm.org/D6992