docs: add classifiers to setup.cfg, fix/update readme badges

[wpbonelli]

• specify compatible OS and Python3 versions in setup.cfg
• fix/update README.md badges
  • fix broken Anaconda badge
  • add Anaconda badges for platform & latest release
  • add PyPI badges for status and supported Python versions

[codecov]

Codecov Report

Merging #1600 (e7ef7ab) into develop (d1a7155) will decrease coverage by 1.2%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           develop   #1600     +/-   ##
=========================================
- Coverage     72.7%   71.5%   -1.3%     
=========================================
  Files          251     251             
  Lines        54449   53102   -1347     
=========================================
- Hits         39614   37973   -1641     
- Misses       14835   15129    +294     

Impacted Files	Coverage Δ
flopy/mf6/modflow/mfgwtgwt.py	33.3% <0.0%> (-29.8%)	⬇️
flopy/mf6/modflow/mfgwfcsub.py	39.0% <0.0%> (-28.2%)	⬇️
flopy/mf6/modflow/mfgwtlkt.py	45.7% <0.0%> (-24.6%)	⬇️
flopy/mf6/modflow/mfgwtmwt.py	45.7% <0.0%> (-24.6%)	⬇️
flopy/mf6/modflow/mfgwtsft.py	45.7% <0.0%> (-24.6%)	⬇️
flopy/mf6/modflow/mfgwtuzt.py	45.7% <0.0%> (-24.6%)	⬇️
flopy/mf6/modflow/mfgwtcnc.py	46.1% <0.0%> (-23.5%)	⬇️
flopy/mf6/modflow/mfgwtsrc.py	46.1% <0.0%> (-23.5%)	⬇️
flopy/mf6/modflow/mfgwfapi.py	47.3% <0.0%> (-22.4%)	⬇️
flopy/mf6/modflow/mfgwtdisu.py	46.5% <0.0%> (-22.0%)	⬇️
... and 85 more