Remove pytest-openfiles, fix unit string comparison in doctests

[pllim]

Description

This pull request is to:

• Handle Avoid resorting units when converting to strings astropy/astropy#14439
• Fix ANN: Please migrate away from pytest-openfiles #349

[github-actions]

Thank you for your contribution! 🌌 This checklist is meant to remind the package maintainers who will review this pull request of some common things to look for.

• Do the proposed changes actually accomplish desired goals?
• Do the proposed changes follow the STScI coding guidelines?
• Are tests added/updated as required? If so, do they follow the STScI testing guidelines?
• Are docs added/updated as required?
• Is rebase and/or squash necessary? If so, please provide the author with appropriate instructions.
• Did the CI pass? If no, are the failures related?
• Is a change log needed?
• Is a milestone set? Milestone must be set but we cannot check for it on Actions; do not let the green checkmark fool you.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (6c2bd36) 97.09% compared to head (e50ef21) 97.09%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #350   +/-   ##
=======================================
  Coverage   97.09%   97.09%           
=======================================
  Files          17       17           
  Lines        2030     2032    +2     
=======================================
+ Hits         1971     1973    +2     
  Misses         59       59           

Impacted Files	Coverage Δ
synphot/blackbody.py	73.33% <100.00%> (+0.45%)	⬆️
synphot/specio.py	99.09% <100.00%> (+<0.01%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.