pythonPackages.keystoneauth1: fix substituteInPlace

pkgs/development/python-modules/keystoneauth1/default.nix

···
 
   postPatch = ''
     sed -i 's@python@${python.interpreter}@' .testr.conf
-    substituteInPlace requirements.txt --replace "argparse"
+    substituteInPlace requirements.txt --replace "argparse" ""
   '';
 }