fix flaky test_backfill_monthly_metrics_for_site by freezing time …

`test_backfill_monthly_metrics_for_site` seems to fail sometimes depending on the time it's being run. using [pytest-freezegun](https://pypi.org/project/pytest-freezegun/) to freeze the time and make sure it returns consistent results.

devsite/requirements/test.txt

@@ -0,0 +1,2 @@
+# Shared testing requirements
+pytest-freezegun==0.4.2

tests/test_backfill.py

@@ -72,6 +72,7 @@ def backfill_test_data(db):
     )
 
 
+@pytest.mark.freeze_time('2019-09-01 12:00:00')
 def test_backfill_monthly_metrics_for_site(backfill_test_data):
     """Simple coverage and data validation check for the function under test
 

tox.ini

@@ -29,6 +29,7 @@ deps =
 	hawthorn_multisite: -r{toxinidir}/devsite/requirements/hawthorn_multisite.txt
 	juniper_community: -r{toxinidir}/devsite/requirements/juniper_community.txt
 	juniper_multisite: -r{toxinidir}/devsite/requirements/juniper_multisite.txt
+	-r{toxinidir}/devsite/requirements/test.txt
 
 whitelist_externals =
 	git