Remove Python 2 support, misc clean-ups

[pllim]

Fix #59

Close #65

Companion to spacetelescope/synphot_refactor#185

• Test against synphot dev after Remove Python 2 support, misc clean-ups synphot_refactor#185 is merged.

[codecov]

Codecov Report

Merging #67 into master will increase coverage by 0.71%.
The diff coverage is 93.93%.

@@            Coverage Diff             @@
##           master      #67      +/-   ##
==========================================
+ Coverage   80.61%   81.32%   +0.71%     
==========================================
  Files          13       13              
  Lines        1630     1585      -45     
==========================================
- Hits         1314     1289      -25     
+ Misses        316      296      -20

Impacted Files	Coverage Δ
stsynphot/commissioning/__init__.py	0% <ø> (ø)	⬆️
stsynphot/commissioning/utils.py	0% <ø> (ø)	⬆️
stsynphot/spark.py	76.47% <0%> (-0.15%)	⬇️
stsynphot/spparser.py	88.88% <100%> (+0.62%)	⬆️
stsynphot/config.py	98.11% <100%> (+3.19%)	⬆️
stsynphot/tables.py	93.54% <100%> (+1.71%)	⬆️
stsynphot/observationmode.py	96.61% <100%> (+0.77%)	⬆️
stsynphot/wavetable.py	97.4% <100%> (+2.34%)	⬆️
stsynphot/spectrum.py	99.34% <100%> (+1.25%)	⬆️
stsynphot/catalog.py	98.19% <100%> (+1.67%)	⬆️
... and 2 more

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