Upgrade PyFxA and add tests to confirm that fxa-generation fields are correctly handled.

[tublitzed]

Once this issue is done, we can jump into the changes we need to make here in the tokenserver.

To support the larger efforts to get the FxA Rust Component into Desktop, we first need to get Sync completely migrated away from BrowserID over to OAuth.

[rfk]

Re: "confirm that generation fields are correctly handled", we need to add a bit of logic so that if the OAuth verifier returns a generation field, then it gets reflected as fxa-generation in the idpClaims dict here:

• tokenserver/tokenserver/verifiers.py

  Line 238 in f038656

  'idpClaims': {},

And add a corresponding test in this file:

• https://github.com/mozilla-services/tokenserver/blob/master/tokenserver/tests/test_oauth_verifier.py

The value of fxa-generation should be an int, not a string.

For completeness we could also consider updating the mock verifier in the service tests to sometimes return a generation number, but I don't know how much churn that would induce:

• tokenserver/tokenserver/tests/test_service.py

  Line 114 in f038656

  def mock_oauth_verifier(self, response=None, exc=None):