ENH: Add pytest_extra_requires to conditionally ignore paths / skip functions unless requirements are met

[ev-br]

fixes gh-197

dt_config = DTConfig()

dt_config.pytest_extra_requires = {
   "path/to/file" : "cupy>=2.3.4",
  "full.function.name": "cupy"
}

Potential follow-ups

• spin-made dev builds do not play nice with packaging.requirements.version. Can add a fallback to __version__. This is only relevant if a spin-built package is a dependency, so this should be very rare. Probably not worth bothering until a there's a real use case.
• allow setting per-doctest requirements: {"module.func": "cupy>2.3.4"}, similar to pytest_extra_ignore

[ev-br]

@flying-sheep would you be interested in giving this a spin to see if this addresses your request in gh-197?

[flying-sheep]

Looks great, only things can have multiple requirements, so that should be handled.

[ev-br]

multiple requirements,

Thanks! Your edits are in.

Adding dt_config.pytest_extra_requires ={"scipy.linalg.solve" : ["cupy", "numpy>2"]} seems to do just what it says on the tin.
This is a pure enhancement and has no backwards compatibility concerns. I'll keep it open for a short while to see if there are further comments, then merge.

[ev-br]

No further comments, so let's land this and iterate as necessary. Thanks a whole lot @flying-sheep !