mailman/python: remove obsolete almebic override, add explanation why this...

python3.override {

  packageOverrides = lib.composeExtensions

    (self: super: {

      # does not find tests

      alembic = super.alembic.overridePythonAttrs (oldAttrs: {

        doCheck = false;

      });

      /*

        This overlay can be used whenever we need to override

        dependencies specific to the mailman ecosystem: in the past

        this was necessary for e.g. psycopg2[1] or sqlalchemy[2].

        In such a large ecosystem this sort of issue is expected

        to arise again. Since we don't want to clutter the python package-set

        itself with version overrides and don't want to change the APIs

        in here back and forth every time this comes up (and as a result

        force users to change their code accordingly), this empty overlay

        is kept on purpose.

        [1] 72a14ea563a3f5bf85db659349a533fe75a8b0ce

        [2] f931bc81d63f5cfda55ac73d754c87b3fd63b291

      */

    })

    overlay;

}