This ticket improves a few things about how doctest treats pytest.

• Introduce the option to specify in function doc TESTS: pytest. This marks the function as tested for the sage-coverage command. Sometimes (but rarely) you have private or protected methods that don't need a proper "EXAMPLES" doctest. In these cases, one can decide to either write traditional TESTS or use pytest to test the method. In the later case, sage-coverage would mark the function as not tested as it's not aware of pytest; so we have to specify explicitly that we use pytest to cover this method. One could even argue that private/protected methods shouldn't be directly tested at all as they are implementation details; but let's not open this discussion.

• Remove _test.py files from doctest coverage. By our convention, these are pytest files and thus almost never have (extensive) doctests.

• To make this work, existing xyz_test.py files that are not using pytest are renamed to test_xyz.py.

CC: @mkoeppe @fchapoton @egourgoulhon @tornaria

Component: doctest coverage

Author: Tobias Diez

Branch/Commit: 01f376c

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/32975