Support Key Stretching Version 2

[dschom]

Because:

Our APIs now support a new key stretching approach (ie key stretching V2)

This Commit:

• Adds support for key stretching V2
  • The salt is now a random value and not the user's email
  • There are now 650000 iterations during the stretch instead of 1000.
• Test have been parameterized to run with both V1 and V2 key stretch approaches
• Effort has been taken to ensure that V1 approach still works
• Key stretch v1 is still the default mode. In order to activate V2, the client must be provided the key_stretch_version=2 argument.

Extra cleanup:

• Tests work again!
• Support for python 2 dropped
• Python 2 ci tests removed
• Adds doc about setup and running tests
• Updates references to Firefox Accounts to be Mozilla Accounts
• Unpins versions in dev-requirements, so libraries can update
• Updates 'stable' urls to point at staging. *.lcip.org are no longer valid
• Fixes bug in test tear down, so that an auth token is provided to /account/destroy
• Skips test_resend_verify_code, which was getting consistently rate limited.
• Skips tests with unsupported endpoints (e.g. /certificate/sign has been deprecated)

Issue that this pull request solves

Closes: FXA-9008

Checklist

• [x] My commit is GPG signed.
• [x] If applicable, I have modified or added tests which pass locally.
• [x I have added necessary documentation (if appropriate).