Non-standard container classes are now maintained

[fwkoch]

For example, previously if you used an OrderedDict for a properties.Dictionary, it would be coerced to a dict. Now, these classes are maintained.

Container classes still do not allow duck-typing - i.e. you still have to use a subclass of dict, not just something that implements __getitem__, __setitem__, etc.

This addresses @bsmithyman's comment here: #188 (review)

[codecov]

Codecov Report

Merging #221 into dev will decrease coverage by 0.03%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##              dev     #221      +/-   ##
==========================================
- Coverage   95.08%   95.04%   -0.04%     
==========================================
  Files          15       15              
  Lines        2114     2120       +6     
==========================================
+ Hits         2010     2015       +5     
- Misses        104      105       +1

Impacted Files	Coverage Δ
properties/base/containers.py	88.66% <90.9%> (-0.1%)	⬇️

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