Upgrade respx for httpx 0.23.1+ compatibility

See release notes at https://github.com/lundberg/respx/releases/tag/0.20.1

Author: phillipuniverse

tox.ini

@@ -270,7 +270,7 @@ deps =
   httpx18: httpx>=0.18.0,<0.19.0
   httpx18: respx~=0.17.0
   httpx21: httpx>=0.19.0
-  httpx21: respx~=0.19.0
+  httpx21: respx~=0.20.1
 
   ; FIXME: add coverage testing
   ; FIXME: add mypy testing