`pytest-doctestplus`

[ev-br]

We had several discussions about combining scipy-doctest and pytest-doctestplus. While the strategy is still TBD, previous work and discussions are scattered all over.
So the first order of business is to collect them together. A minimal, potentially incomplete list:

• An early discussion in the NumPy tracker (predates scipy-doctest as a pytest plugin): DEV: --doctests option to runtests.py doesn't do anything numpy/numpy#21070 (comment)
• A more recent NumPy discussion, when converting the NumPy usage to scipy-doctest:
• refguide-check with pytest as a runner numpy/numpy#26604 (comment)
• scikit-learn's adoption: DOC Investigate scipy-doctest for more convenient doctests scikit-learn/scikit-learn#29027

Cross-ref gh-59 for a discussion about the third option in the "better doctesting" space, xdoctest.

At least in the medium term, while the grand strategy is TBD, we are using the scipy-doctest pluggable and configurable architecture to bring it to feature parity with other tools (and pytest itself). Ongoing work:

• ENH: add ParserKlass key to DTConfig; check xdoctest.DocTestParser #174 is a WIP attempt to use xdoctest's Parser;
• Doctestplus adapter #188 is a demo of using the pytest-doctestplus's Checker;
• ENH: Add pytest_extra_requires to conditionally ignore paths / skip functions unless requirements are met #204 implements pytest_extra_requires, requested in DoctestPlus and optional dependencies #197
• Stop skipping tests #198 (comment) tracks a change to make the handling of doctests vs "regular tests" controllable and align with the rest of the ecosystem.

Let's use this issue to (i) work out the long-term path forward, and (ii) flag missing features or quirks.