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.

app/letter_branding/letter_branding_schema.py

@@ -6,5 +6,5 @@ post_letter_branding_schema = {
         "name": {"type": ["string", "null"]},
         "filename": {"type": ["string", "null"]},
     },
-    "required": ("name", "filename")
+    "required": ["name", "filename"]
 }