retire old parent

[Martin R]

Dear all,

after a lot of work, mainly by Frédéric, there are now two pull requests that retire all of the old parent structure, that is, `Parent` in `parent_old.py`,  `ParentWithBase` and `ParentWithGens`.  These have been deprecated in Commit 26da9c4 by robertwb

on Oct 16, 2008.  It was not completely trivial to remove them.

I'd especially like to encourage you to test https://github.com/sagemath/sage/pull/42245, because it touches on the interfacing with other systems, in particular FriCAS, macaulay2, etc.

Of course, I'm grateful for any comments.

Have fun,

Martin