pypy3Packages.greenlet: Set null for pypi

Greenlet is provided as a built-in package on pypy and uninstalling and
replacing it does not work.

Set it to null, to make it disappear as a dependency, instead of marking
it as disabled, which also disables reverse dependencies.

Co-Authored-By: Jean-Paul Calderone <exarkun@twistedmatrix.com>

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

@@ -12,7 +12,6 @@ buildPythonPackage rec {
   pname = "greenlet";
   version = "2.0.2";
   format = "setuptools";
-  disabled = isPyPy; # builtin for pypy
 
   src = fetchPypi {
     inherit pname version;

pkgs/top-level/python-packages.nix

@@ -4180,7 +4180,8 @@ self: super: with self; {
 
   greeneye-monitor = callPackage ../development/python-modules/greeneye-monitor { };
 
-  greenlet = callPackage ../development/python-modules/greenlet { };
+  # built-in for pypi
+  greenlet = if isPyPy then null else callPackage ../development/python-modules/greenlet { };
 
   grequests = callPackage ../development/python-modules/grequests { };