pythonPackages.certifi: does not support python2

Support was officially dropped in
https://github.com/certifi/python-certifi/commit/5efdd48f719d9c3c7c8f9a812da2256d088eab78

(cherry picked from commit 7748af9fff7867dc7c413c762b36793537b6cf47)

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

@@ -1,5 +1,6 @@
 { lib
 , buildPythonPackage
+, isPy27
 , fetchFromGitHub
 , pytestCheckHook
 }:
@@ -8,6 +9,8 @@ buildPythonPackage rec {
   pname = "certifi";
   version = "2021.05.30";
 
+  disabled = isPy27;
+
   src = fetchFromGitHub {
     owner = pname;
     repo = "python-certifi";