OpenAPI schema generation: Don't convert all enums into str

[songzhi]

Before doing this,the type of values in enum could be at odds with type field.

[codecov]

Codecov Report

Merging #906 into master will decrease coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #906      +/-   ##
==========================================
- Coverage     100%   99.92%   -0.08%     
==========================================
  Files         293      293              
  Lines        7692     7701       +9     
==========================================
+ Hits         7692     7695       +3     
- Misses          0        6       +6

Impacted Files	Coverage Δ
fastapi/openapi/models.py	100% <100%> (ø)	⬆️
fastapi/concurrency.py	77.77% <0%> (-22.23%)	⬇️
tests/test_filter_pydantic_sub_model.py	100% <0%> (ø)	⬆️
fastapi/utils.py	100% <0%> (ø)	⬆️

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[tiangolo]

Thanks! Good point 🔍 🚀 I updated it to allow any type as subtype of List.

Thanks for your contribution! 🍰