run-tests: add --with-python3 to define a Python 3 interpreter Currently, very few parts of Mercurial run under Python 3, notably the test harness. We want to write tests that run Python 3. For example, we want to extend test-check-py3-compat.t to parse and load Python files. However, we have a problem: finding appropriate files requires running `hg files` and this requires Python 2 until `hg` works with Python 3. As a temporary workaround, we add --with-python3 to the test harness to allow us to define the path to a Python 3 interpreter. This interpreter is made available to the test environment via $PYTHON3 so tests can run things with Python 3 while the test harness and `hg` invocations continue to run from Python 2. To round out the feature, a "py3exe" hghave check has been added.