pythonPackages.google-auth: add missing dependency on six

google-auth depends on six as indicated here:
https://github.com/googleapis/google-auth-library-python/blob/65074d3fbe488b95923dfa8335d071fa90bf383f/setup.py

It only builds on hydra without it right now,
because another dependency pulls it in as a checkInput:
https://github.com/DavHau/mach-nix/issues/240#issuecomment-786015221

pkgs/development/python-modules/google-auth/default.nix

@@ -12,6 +12,7 @@
 , pytest-localserver
 , responses
 , rsa
+, six
 }:
 
 buildPythonPackage rec {
@@ -23,7 +24,7 @@ buildPythonPackage rec {
     sha256 = "0bmdqkyv8k8n6s8dss4zpbcq1cdxwicpb42kwybd02ia85mh43hb";
   };
 
-  propagatedBuildInputs = [ pyasn1-modules cachetools rsa ];
+  propagatedBuildInputs = [ pyasn1-modules cachetools rsa six ];
 
   checkInputs = [
     flask