Squash a few deprecation warnings

- inspect.getargspec() -> inspect.getfullargspec()
- register pytest.mark.fixture_args
- replace use of DRF's deprecated NullBooleanField
- fix some usage of naive datetimes in the tests
- fix some strings with backslashes that ought to be raw strings

pytest.ini

@@ -12,4 +12,5 @@ markers =
     job_permissions:
     activity_stream_access:
     job_runtime_vars:
+    fixture_args:
 junit_family=xunit2