Upstream wptserve just switched to Python 3 only. That's fine for
web-platform-tests, but it turns out that some marionette harness
tests are also using wptserve and are still on Python 2.
Since fixing marionette harness turns out to be non-trivial and this
blocks other wpt work, this patch does the following:
* Temporarily vendors the last wptserve revision that works with
Python 2 in to testing/web-platform/mozilla/tests/tools/wptserve_py2
* Configures the mach virtualenv to use that copy for Python 2 modules
only.
* Configures the test packaging system to also put that copy in the
common tests zip. Requirements files are updated to use either the
Python 2 version or the Pyhton 3 version as required.
Differential Revision: https://phabricator.services.mozilla.com/D106764
7814585840