Bump pyfxa from 0.6.0 to 0.7.1 (#335)

Bumps [pyfxa](https://github.com/mozilla/PyFxA) from 0.6.0 to 0.7.1.
- [Release notes](https://github.com/mozilla/PyFxA/releases)
- [Changelog](https://github.com/mozilla/PyFxA/blob/master/CHANGES.txt)
- [Commits](https://github.com/mozilla/PyFxA/compare/v0.6.0...v0.7.1)

Signed-off-by: dependabot[bot] <support@dependabot.com>

requirements/py27-tests.txt

@@ -1,6 +1,6 @@
 mock==2.0.0
 mozlog==4.0
-pyfxa==0.6.0
+pyfxa==0.7.1
 pypom==2.2.0
 pytest==4.3.1
 pytest-fxa==1.4.0

requirements/py36-tests.txt

@@ -1,5 +1,5 @@
 mock==2.0.0
-pyfxa==0.6.0
+pyfxa==0.7.1
 pypom==2.2.0
 pytest==4.3.1
 pytest-fxa==1.4.0