Making a `nox -s default` session for all packages. (#4324)
Making a `nox -s default` session for all packages. (#4324)

* Making a `nox -s default` session for all packages.

* Using "default" `nox` session on AppVeyor.

This was 32-bit or 64-bit Python can be used, depending
on which is the active `python` / the active `nox.exe`.

googleapis/python-monitoring,googleapis/python-monitoring