overhaul psi4 detection parsing

[loriab]

Description

The existing psi4 detection has worked great when psi4 is in order and conda environments prevail, but #389 and #292 show that if something is set up a little wrong with psi4, then everything devolves into an IndexError with the existing psi4 detection.

This PR improves matters by

• in the complete-the-psithon-psiapi-pair blocks:
  • only doing string-safe operations on stdout
  • forming clearer error responses
  • catching stderr instead of tossing it and parsing stdout anyways
  • getting rid of env passing. As @Flamefire pointed out, the psi4 module is already in sys.path or which_import wouldn't have worked. unlike #389, code is now passing no env so current env is used (and can provide numpy, etc.). I've tested that there's no need to provide env=os.environ to get current env.
  • checking returncode
  • it was also not showing error messages when psithon found but not psiapi b/c final check was of psithon only. now checking at the weak point
• in the version fn:
  • only doing string-safe operations on stdout
  • catching stderr instead of tossing it and parsing stdout anyways
  • checking returncode

Hopefully this will be more robust. closes #389 closes #391 closes #292

Changelog description

Status

• [x] Code base linted
• [x] Ready to go

[codecov[bot]]

Codecov Report

Merging #418 (a0ce68a) into master (034fd1a) will decrease coverage by 0.17%. Report is 2 commits behind head on master. The diff coverage is 28.00%.

Additional details and impacted files

[loriab]

Yes, if there was any good reason behind that env reset, I haven't been able to reconstruct it. It'll be nice to get good traceback back, rather than remembering that IndexError in psi4 found() means the version/tag is broken.