Fix CI checks by avoiding incompatible Python modules

Avoid installing urllib3>=2 for CI runs as the responses module is not
compatible with that version yet (leading to failing tests because
mocking HTTP responses in not working correctly).

Supposedly getsentry/responses#635 is the
relevant upstream issue.

requirements-dev.txt

@@ -9,3 +9,4 @@ ruamel.yaml
 beautifulsoup4
 black
 jsonschema
+urllib3<2 # responses needs <2, see https://github.com/getsentry/responses/issues/635