Make get_current_financial_year timezone aware

It would have given the wrong answer for the first hour of the 2021
financial year.

This was OK before, because we didn’t need this kind of precision. But
now it could mean someone signing up in the middle of the night getting
the wrong free text message allowance, so we should fix it.

app/utils.py

@@ -334,7 +334,9 @@ def get_template(
 
 
 def get_current_financial_year():
-    now = datetime.utcnow()
+    now = utc_string_to_aware_gmt_datetime(
+        datetime.utcnow()
+    )
     current_month = int(now.strftime('%-m'))
     current_year = int(now.strftime('%Y'))
     return current_year if current_month > 3 else current_year - 1

tests/app/test_utils.py

@@ -16,6 +16,7 @@ from app.utils import (
     generate_next_dict,
     generate_notifications_csv,
     generate_previous_dict,
+    get_current_financial_year,
     get_letter_printing_statement,
     get_letter_validation_error,
     get_logo_cdn_domain,
@@ -657,3 +658,14 @@ def test_merge_jsonlike_merges_jsonlike_objects_correctly(source_object, destina
 ))
 def test_round_to_significant_figures(value, significant_figures, expected_result):
     assert round_to_significant_figures(value, significant_figures) == expected_result
+
+
+@pytest.mark.parametrize('datetime_string, financial_year', (
+    ('2021-01-01T00:00:00+00:00', 2020),  # Start of 2021
+    ('2021-03-31T22:59:59+00:00', 2020),  # One minute before midnight (BST)
+    ('2021-03-31T23:00:00+00:00', 2021),  # Midnight (BST)
+    ('2021-12-12T12:12:12+01:00', 2021),  # Later in the year
+))
+def test_get_financial_year(datetime_string, financial_year):
+    with freeze_time(datetime_string):
+        assert get_current_financial_year() == financial_year