#14793: Use __reduce__ for pickling homsets, and use WithEqualityById for them
Some review changes.
Trac 14793: Add a missing doctest
Improve Parent.__getitem__

- make it work for Cython objects too
- do not catch AttributeError's raised by the method we found
Move __getitem__ from rings.ring to categories.ring
Move is_zero from Ring to Rings.ParentMethods
Make gen_name private
Simplify ZZ.__getitem__
Clean up and simplify Rings.ParentMethods.__getitem__
Further simplify/robustify Rings.ParentMethods.__getitem__
A schizophrenic __getitem__ for matrix rings
Clarify that polynomials over general rings are supported
coercion tutorial: Update some examples...

...to reflect that Rings gained a few parent methods
Improve doc of Rings.EM.__getitem__ and related functions

* More crossrefs to other relevant implementations of__getitem__
* Minor improvements suggested by Nicolas Thiéry in his review of #8389
Fix broken link in doc of Rings.EM.__getitem__
Replaced all occurrences of GPLv3 with GPLv2+

This also modifies the developer's guide which recommended to release
contributions under GPLv3+, see 67a76032. Now it says again GPLv2+.
Trac 11652: fixes degree for multivariate libsingular polynomials
Trac 11652: Review patch for the docstring of degree() for multivariate polynomials
Update cddlib to 094g and tidy the patches a little bit.
Update tests folder for cddlib.