Re-organize imports from collections and collections.abc

[mwtoews]

The main fix addresses an upcoming change:

/path/to/flopy/flopy/mf6/mfbase.py:680:DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3, and in 3.10 it will stop working
    if not isinstance(load_only, collections.Iterable):

The other change is just to simplify an import from a related module.

[codecov]

Codecov Report

Merging #1057 (c07b25d) into develop (943ace4) will increase coverage by 0.979%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##           develop     #1057       +/-   ##
=============================================
+ Coverage   71.769%   72.749%   +0.979%     
=============================================
  Files          225       221        -4     
  Lines        51895     51125      -770     
=============================================
- Hits         37245     37193       -52     
+ Misses       14650     13932      -718     

Impacted Files	Coverage Δ
flopy/mf6/mfbase.py	86.197% <100.000%> (+0.072%)	⬆️
flopy/modflow/mfswi2.py	62.461% <0.000%> (-9.847%)	⬇️
flopy/modflow/mfoc.py	91.732% <0.000%> (-2.953%)	⬇️
flopy/modflow/mfbcf.py	87.939% <0.000%> (-2.011%)	⬇️
flopy/utils/datafile.py	72.811% <0.000%> (-1.383%)	⬇️
flopy/utils/voronoi.py
flopy/mt3d/mtcts.py
flopy/mf6/utils/reference.py
flopy/mf6/utils/testutils.py