Increase version requirement for pytest-cases, unpin pytest

[bhazelton]

Description

Require a higher version of pytest-cases to fix an incompatibility with pytest>=7. Undo the pinning of pytest done in #1138.

Also fix the import of readsav from scipy to address new DeprecationWarning.

Motivation and Context

see #1138.

Types of changes

• Build or continuous integration change

Checklist:

• I have read the contribution guide.
• My code follows the code style of this project.

Build or continuous integration change checklist:

• If required or optional dependencies have changed (including version numbers), I have updated the readme to reflect this.

[codecov]

Codecov Report

Merging #1140 (58ef744) into main (f2b60f7) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1140   +/-   ##
=======================================
  Coverage   99.83%   99.83%           
=======================================
  Files          31       31           
  Lines       15387    15387           
=======================================
  Hits        15361    15361           
  Misses         26       26           

Impacted Files	Coverage Δ
pyuvdata/uvcal/fhd_cal.py	100.00% <100.00%> (ø)
pyuvdata/uvdata/fhd.py	99.66% <100.00%> (ø)

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[plaplant]

Looks great, thanks @bhazelton!