fix(unit): serialization into string not dict

[EribertoLopez]

Issue:

asdict(DispenseColumn) returns {'column': 1, 'volume': {'value': 700.0, 'units': 'microliter'}}

• Unit dataclass is being serialized into a dict but is expected as a str

Solution:

• Adding asdict(obj, dict_factory=unit_as_strings_factory)

[codecov-commenter]

Codecov Report

Merging #389 (67ab4d1) into master (84665a7) will decrease coverage by 0.01%.
The diff coverage is 94.44%.

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@@            Coverage Diff             @@
##           master     #389      +/-   ##
==========================================
- Coverage   84.62%   84.62%   -0.01%     
==========================================
  Files          23       24       +1     
  Lines        4541     4554      +13     
==========================================
+ Hits         3843     3854      +11     
- Misses        698      700       +2     

Impacted Files	Coverage Δ
autoprotocol/version.py	0.00% <0.00%> (ø)
autoprotocol/builders.py	85.73% <100.00%> (ø)
autoprotocol/protocol.py	79.34% <100.00%> (+0.01%)	⬆️
autoprotocol/types/__init__.py	100.00% <100.00%> (ø)
autoprotocol/types/ref.py	94.59% <100.00%> (+0.15%)	⬆️
autoprotocol/unit.py	87.59% <100.00%> (-0.96%)	⬇️

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[joshhacksthings]

lgtm!