Support OpenAPI discriminator

[wyfo]

Closes #220

[codecov-commenter]

Codecov Report

Merging #301 (5f6a7e1) into master (4b50187) will increase coverage by 0.56%.
The diff coverage is 93.53%.

@@            Coverage Diff             @@
##           master     #301      +/-   ##
==========================================
+ Coverage   85.44%   86.00%   +0.56%     
==========================================
  Files          66       67       +1     
  Lines        5675     5888     +213     
  Branches     1170     1225      +55     
==========================================
+ Hits         4849     5064     +215     
+ Misses        607      602       -5     
- Partials      219      222       +3     

Impacted Files	Coverage Δ
apischema/json_schema/types.py	92.68% <ø> (ø)
apischema/deserialization/methods.py	84.80% <72.00%> (-0.31%)	⬇️
apischema/serialization/methods.py	87.63% <88.46%> (+1.54%)	⬆️
apischema/discriminators.py	95.23% <95.23%> (ø)
apischema/json_schema/schema.py	85.59% <95.23%> (+1.02%)	⬆️
apischema/serialization/__init__.py	84.55% <97.05%> (+2.22%)	⬆️
apischema/__init__.py	84.21% <100.00%> (+0.42%)	⬆️
apischema/conversions/visitor.py	95.23% <100.00%> (+8.28%)	⬆️
apischema/deserialization/__init__.py	98.97% <100.00%> (+0.10%)	⬆️
apischema/json_schema/refs.py	89.58% <100.00%> (+1.34%)	⬆️
... and 4 more

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