Remove cast to builtin types in deserialization methods

[wyfo]

Cast was added in order allow Cython using optimized operations on
containers. However, quick benchmark showed that it was in fact
surprisingly slower for small lists.

[codecov-commenter]

Codecov Report

Merging #348 (de0b76d) into master (8349491) will decrease coverage by 0.01%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master     #348      +/-   ##
==========================================
- Coverage   86.22%   86.20%   -0.02%     
==========================================
  Files          67       67              
  Lines        5886     5878       -8     
  Branches     1221     1221              
==========================================
- Hits         5075     5067       -8     
  Misses        593      593              
  Partials      218      218              

Impacted Files	Coverage Δ
apischema/deserialization/methods.py	84.80% <84.61%> (-0.19%)	⬇️

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