pythonPackages: set mainProgram to pname by default

Calling `nix run poetry` or another python package usually fails
because of the "pythonX" prefix in name.
Adjust mainProgram to ignore that prefix.

(cherry picked from commit b59875ef23dabddceda3673a7559ebbede9ab6aa)

pkgs/development/interpreters/python/mk-python-derivation.nix

@@ -176,6 +176,8 @@ let
       # default to python's platforms
       platforms = python.meta.platforms;
       isBuildPythonPackage = python.meta.platforms;
+    } // lib.optionalAttrs (attrs?pname) {
+      mainProgram = attrs.pname;
     } // meta;
   } // lib.optionalAttrs (attrs?checkPhase) {
     # If given use the specified checkPhase, otherwise use the setup hook.