gcovr: Add alias to pythonPackages.gcovr

Since gcovr is most likely to be used as a script instead of a library,
we provide an alias to python3Packages.gcovr. We still keep
python2/3Packages.gcovr in case somebody really wants to import it as a
library, which can not entirely be ruled out.

pkgs/top-level/all-packages.nix

@@ -10383,6 +10383,8 @@ in
 
   gcab = callPackage ../development/libraries/gcab { };
 
+  gcovr = with python3Packages; toPythonApplication gcovr;
+
   gcr = callPackage ../development/libraries/gcr { };
 
   gdl = callPackage ../development/libraries/gdl { };