refactor(mf6): update DFNS for mf6.4.3, regen/reformat .py files

[wpbonelli]

• update DFNs from mf6 repository for mf6.4.3 release
• regenerate mf6 component classes
• reformat mf6 component classes
• reformat some tests and notebooks
• add DFN backup dir to .gitignore

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (154ba16) 73.0% compared to head (088f37e) 73.0%.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #2095   +/-   ##
=======================================
  Coverage     73.0%   73.0%           
=======================================
  Files          259     259           
  Lines        59357   59364    +7     
=======================================
+ Hits         43352   43359    +7     
  Misses       16005   16005           

Files	Coverage Δ
flopy/mf6/modflow/mfems.py	72.7% <ø> (ø)
flopy/mf6/modflow/mfgnc.py	90.9% <ø> (ø)
flopy/mf6/modflow/mfgwf.py	100.0% <ø> (ø)
flopy/mf6/modflow/mfgwfapi.py	69.6% <ø> (ø)
flopy/mf6/modflow/mfgwfbuy.py	74.1% <ø> (ø)
flopy/mf6/modflow/mfgwfchd.py	100.0% <100.0%> (ø)
flopy/mf6/modflow/mfgwfcsub.py	67.5% <ø> (ø)
flopy/mf6/modflow/mfgwfdis.py	100.0% <ø> (ø)
flopy/mf6/modflow/mfgwfdisu.py	100.0% <ø> (ø)
flopy/mf6/modflow/mfgwfdisv.py	100.0% <ø> (ø)
... and 61 more