Bump jsonschema from 3.2.0 to 4.4.0

The big breaking change for our code (not mentioned in the changelog) is
that the built-in validator for the `date-time` format now requires the
`rfc3339-validator` package instead of the `strict-rfc3339` package.
This updates the requirements file to use `rfc3339-validator`. Without
this change, wrong `date-time` formats would always silently pass validation.

tests/app/complaint/test_complaint_rest.py

@@ -87,5 +87,4 @@ def test_get_complaint_with_invalid_data_returns_400_status_code(client):
     )
 
     assert response.status_code == 400
-    assert response.json['errors'][0]['message'] == 'start_date time data {} does not match format %Y-%m-%d'.format(
-        start_date)
+    assert response.json['errors'][0]['message'] == 'start_date month must be in 1..12'

tests/app/platform_stats/test_rest.py

@@ -48,8 +48,7 @@ def test_get_platform_stats_validates_the_date(admin_request):
         _expected_status=400
     )
 
-    assert response['errors'][0]['message'] == 'start_date time data {} does not match format %Y-%m-%d'.format(
-        start_date)
+    assert response['errors'][0]['message'] == 'start_date month must be in 1..12'
 
 
 @freeze_time('2018-10-31 14:00')