It looks like Mac workers are already using 3.7.
However, Linux workers are still mostly Python 3.6, and there's some Windows workers still on 3.6 (perhaps they're using out-of-date MozillaBuild versions? Modern MozillaBuild uses Python 3.7).

A couple notes for this ticket:

• Some Python tests may start imploding due to requiring Python < 3.7 behaviour
  • For example, the test_vendor.py test will require a ./mach vendor python on 3.7 to be committed first.
• Hopefully other breakage will be minimal, since OSX workers are already on 3.7.