MNT: Unpin tox, replace relative imports

[pllim]

Description

This pull request is to

• fix MNT: Unpin tox<4 when we can #171
• fix MNT: Replace relative with absolute imports #152

[github-actions]

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[codecov]

Codecov Report

Base: 84.88% // Head: 84.85% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (6f01f2d) compared to base (7a4f2da).
Patch coverage: 94.28% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #173      +/-   ##
==========================================
- Coverage   84.88%   84.85%   -0.04%     
==========================================
  Files          13       13              
  Lines        1628     1637       +9     
==========================================
+ Hits         1382     1389       +7     
- Misses        246      248       +2     

Impacted Files	Coverage Δ
stsynphot/__init__.py	85.71% <80.00%> (-14.29%)	⬇️
stsynphot/catalog.py	98.23% <100.00%> (ø)
stsynphot/commissioning/utils.py	29.82% <100.00%> (ø)
stsynphot/config.py	98.11% <100.00%> (ø)
stsynphot/observationmode.py	96.68% <100.00%> (ø)
stsynphot/spark.py	76.47% <100.00%> (ø)
stsynphot/spectrum.py	99.37% <100.00%> (ø)
stsynphot/spparser.py	89.31% <100.00%> (ø)
stsynphot/stio.py	96.35% <100.00%> (ø)
stsynphot/tables.py	93.47% <100.00%> (ø)
... and 1 more

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