Use pendulum to traverse mock reporting dates

Previously, subtracting 29 days could result in the same month listed
twice (for example, subtracting 29 days from Nov 30, would result in two
Novembers listed).

tests/routes/portfolios/test_portfolios_index.py

@@ -88,4 +88,4 @@ def test_portfolio_reports_with_mock_portfolio(client, user_session):
     )
     assert response.status_code == 200
     assert portfolio.name in response.data.decode()
-    assert "$237,617.00 Total spend to date" in response.data.decode()
+    assert "$251,626.00 Total spend to date" in response.data.decode()