ENH: use PYTHONWARNDEFAULTENCODING=1 to fix optional encoding warnings in Python 3.10+

[Zac-HD]

Under Python 3.11, though for some reason I haven't investigated not 3.10,

import numpy.testing

triggers the following EncodingWarning

$ python -X warn_default_encoding demo.py
numpy/testing/_private/utils.py:1251: EncodingWarning: 'encoding' argument not specified.
  output = subprocess.run(cmd, capture_output=True, text=True)

It would be great if Numpy added this to CI and fixed however many errors that turns up; with Pytest 7.4.0 and later running clean it's now reasonably practical to do so.

You may want a two-stage approach where you fix all the current cases in Numpy first (like HypothesisWorks/hypothesis#3619), and then revisit enforcing it in CI once upstream projects have cleaned up their own warnings (like HypothesisWorks/hypothesis#3691, where I hit this issue).

[mattip]

I see. We need to add encoding="utf-8" a bit everywhere to make that warning go away, and remove it for python 3.15+, according to PEP 686. That warning was added for python3.11, so one the 3.11+ CI runs should add the env variable.

[Zac-HD]

The warning exists from Python 3.10, though there were some related small changes in 3.11.

We don't actually need to remove the encoding arguments in 3.15 - as I understand it Python will then default to using uft-8 by default, but end-users can still disable that so libraries should probably keep specifying encoding when we know which of utf-8 or locale is preferred.