[weblate] Pin version of user-agents (tails#17377)

The version in the Pypi JSON is "2.1" but there's no such tag in the
repo and our dep update script does not currently account for such
differences.

scripts/weblate/override.yaml

@@ -46,6 +46,9 @@ pip:
     repo_type: hg
     additional_condition:
       - <3.0  # Bigger versions require Python >=3.6
+  user-agents:
+    additional_condition:
+      - ==2.1.0  # Current version in Pypi is "2.1" and there's no such tag in the repo.
   rjsmin:
     url: https://github.com/ndparker/rjsmin
   translate-toolkit: