refactor: start using __slots__

[chrislemke]

https://stackoverflow.com/questions/472000/usage-of-slots

[codecov]

Codecov Report

Merging #89 (4c8aad1) into develop (34a9863) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop      #89      +/-   ##
===========================================
+ Coverage    99.41%   99.48%   +0.06%     
===========================================
  Files            9        9              
  Lines          517      583      +66     
===========================================
+ Hits           514      580      +66     
  Misses           3        3              

Impacted Files	Coverage Δ
src/sk_transformers/__init__.py	100.00% <100.00%> (ø)
src/sk_transformers/base_transformer.py	100.00% <100.00%> (ø)
src/sk_transformers/datetime_transformer.py	100.00% <100.00%> (ø)
src/sk_transformers/deep_transformer.py	100.00% <100.00%> (ø)
src/sk_transformers/encoder_transformer.py	100.00% <100.00%> (ø)
src/sk_transformers/generic_transformer.py	99.50% <100.00%> (+0.12%)	⬆️
src/sk_transformers/number_transformer.py	100.00% <100.00%> (ø)
src/sk_transformers/string_transformer.py	98.55% <100.00%> (+0.17%)	⬆️

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