Bug 1685573 [wpt PR 27081] - Make --py3 the default for 'wpt', a=testonly

Bug 1685573 [wpt PR 27081] - Make --py3 the default for 'wpt', a=testonly

Automatic update from web-platform-tests
Make --py3 the default for 'wpt' (#27081)

As per
https://github.com/web-platform-tests/rfcs/blob/master/rfcs/py_3.md,
step #2 of the transition to Python 3-only is to make 'wpt ...' commands
run in Python 3 by default.

Passing --py2 will now be necessary to run under Python 2. (Until ~Feb
2021, when we will remove py2 support entirely).

This does affect some CI runs. Cases where they already specified py3
will remain py3. Cases which are designed to run under py2 had `--py2`
added. Cases that didn't currently specify and aren't version specific
are upgraded from py2 to py3 (one example is Azure Pipelines Mac
infrastructure tests.)

Some Azure Pipelines helper scripts are used for both py2 and py3 tasks.
As a simple way to keep them working, `--py2` is used for them as it is
always available.
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wpt-commits: 61d85c8f90cad174dd83d97399b894554705915e
wpt-pr: 27081