pythonPackages.requests: fix dependencies for python2

* brotlicffi does not support python 2, so only include it as a build
  input of requests if requests is being built for python 3.
  otherwise, fall back to brotli.

* requests 2.26 release notes specify that charset-normalizer is only
  for python 3, while chardet is only for python 2.  correct build
  inputs to reflect this.  (charset-normalizer does not build with
  python 2.)

fixes #138584

pkgs/development/python-modules/requests/default.nix

@@ -36,10 +36,7 @@ buildPythonPackage rec {
   '';
 
   propagatedBuildInputs = [
-    brotlicffi
     certifi
-    charset-normalizer
-    chardet
     idna
     urllib3
   ] ++ lib.optionals (isPy3k) [
@@ -47,6 +44,7 @@ buildPythonPackage rec {
     charset-normalizer
   ] ++ lib.optionals (isPy27) [
     brotli
+    chardet
   ];
 
   checkInputs = [