Allow return type of endpoint to be response model

[JarroVGIT]

Based on the discussion here:

Fixes #5215

This would allow the developer to choose between using something like

@app.get("/", response_model=MyModel)
def root():
    pass

and

@app.get("/")
def root() -> MyModel:
    pass

The latter is more idiomatic. In this PR, the reponse_model= method takes precedence, as all documentation is in that style. That means if both are set, then the return type will be ignored.

[codecov]

Codecov Report

Merging #5216 (d790941) into master (5c576e4) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##            master    #5216      +/-   ##
===========================================
- Coverage   100.00%   99.98%   -0.02%     
===========================================
  Files          532      533       +1     
  Lines        13684    13728      +44     
===========================================
+ Hits         13684    13726      +42     
- Misses           0        2       +2     

Impacted Files	Coverage Δ
fastapi/routing.py	100.00% <100.00%> (ø)
fastapi/utils.py	100.00% <100.00%> (ø)
tests/test_endpoint_response_model_return_type.py	100.00% <100.00%> (ø)
fastapi/concurrency.py	90.00% <0.00%> (-10.00%)	⬇️

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[github-actions]

📝 Docs preview for commit 8a07b01 at: https://62e6d9aef71fda183aa15bc0--fastapi.netlify.app

[github-actions]

📝 Docs preview for commit d6efeb6 at: https://62e6dfaef9d4021787025f37--fastapi.netlify.app

[github-actions]

📝 Docs preview for commit d790941 at: https://62e6e959ab662b1a31cc19ed--fastapi.netlify.app

[JarroVGIT]

This has been discussed and rejected in #101 :(