Developer guide: internal classes are not subject to deprecation policy

[Kwankyu Lee]

Hi,

In https://github.com/sagemath/sage/issues/38211, we are discussing the details of the deprecation policy.

The main issue is to clarify what are "internal" classes. This is the idea:

(C) A class whose name has an underscore anywhere is internal, and is not subject to the deprecation policy.

Please come and leave your opinions. Thanks.