Fixed python 3.8.7 compatibility

[vinayinvicible]

• Upgraded starlette to 0.14.2
• Added UJSONResponse into the project for backward compatibility

Ref: encode/starlette#1126, encode/starlette#901

[ycd]

Duplicate of #2335.

[github-actions]

📝 Docs preview for commit 50d7072 at: https://6026a7e0751b7600919e376d--fastapi.netlify.app

[codecov]

Codecov Report

Merging #2798 (50d7072) into master (a7a353e) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2798   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          243       244    +1     
  Lines         7419      7437   +18     
=========================================
+ Hits          7419      7437   +18     

Impacted Files	Coverage Δ
fastapi/responses.py	100.00% <100.00%> (ø)
..._tutorial/test_custom_response/test_tutorial001.py	100.00% <100.00%> (ø)

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[vinayinvicible]

@ycd
sorry my bad

[tiangolo]

Thanks for the help here @ycd ! 👏 🙇

Thanks for reporting back and closing it @vinayinvicible 👍

Sorry for the long delay! 🙈 I wanted to personally address each issue/PR and they piled up through time, but now I'm checking each one in order.