Limit httpx to <0.20.0

This one limits httpx to <0.20.0 in order to handle breaaking
change for CloudSQL provider.

apache#20088

We do not add it in main /2.3 because we will fix the problem
in main and release new provider then.

setup.cfg

@@ -109,7 +109,8 @@ install_requires =
     flask-wtf>=0.14.3, <0.15
     graphviz>=0.12
     gunicorn>=20.1.0
-    httpx
+    # We need to limit httpx until https://github.com/apache/airflow/issues/20088 is fixed
+    httpx<0.20.0
     importlib_metadata>=1.7;python_version<"3.9"
     importlib_resources~=5.2;python_version<"3.7"
     # Required by vendored-in connexion