refactor: upgrade code to >=python3.7 (pre-commit)

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2189>

api/tests/utils.py

@@ -10,4 +10,4 @@ def to_api_date(value):
     if isinstance(value, datetime.date):
         f = rest_fields.DateField()
         return f.to_representation(value)
-    raise ValueError("Invalid value: {}".format(value))
+    raise ValueError(f"Invalid value: {value}")