mailmanPackages.python: allow changing python package-set used for mailman

When having a patch for a python module that should only be used for
mailman, but for nothing else, it's now possible to apply it like this:

    self: super: {
      mailmanPackages = super.mailmanPackages.extend (mailmanSelf: mailmanSuper: {
	python3 = mailmanSuper.python3.override {
	  overlay = pythonSelf: pythonSuper: {
	    psycopg2 = /* ... */;
	  };
	};
      });
    }

The underlying issue is that the `packageOverrides`-mechanism of
`pkgs.python3` doesn't compose, so an optional overlay is manually
applied to the `python3` used for mailman.

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