Propagate callable's return type annotation for Depends/Security

[kernc]

I was struck by the following paragraph in tutorial for Depends: https://github.com/tiangolo/fastapi/blob/51e920e2fcf3e2a715081f28e3187549f2bf531a/docs/en/docs/tutorial/dependencies/classes-as-dependencies.md#L138-L150

I though that's easily solvable with typing.TypeVar, and this PR serves as a untested proof-of-concept, further corroborated only by the fact that the following short snippet passes mypy checks:

from typing import Callable, TypeVar

T = TypeVar('T')

class C:
    ...

def foo(x: Callable[..., T]) ->  T:
    return bar()

def bar():
    return ...

c: C = foo(C)   # <-- no error here

This alleviates the need for repeating the dependency (e.g. CommonQueryParams) in the annotation. :crossed_fingers:

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a7a353e) 100.00% compared to head (8f5174c) 100.00%. Report is 2021 commits behind head on master.

Additional details and impacted files

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##            master     #2473   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          243       243           
  Lines         7419      7420    +1     
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+ Hits          7419      7420    +1     

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[kernc]

While an obvious example of laziness and poor judgement, with tests still passing, the proof-of-concept seems to work. :sweat_smile:

What would extend this PoC into a full-blown PR is:

• [ ] adjusting above documentation paragraph,
• [ ] using the pattern elsewhere where/if useful.

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[kernc]

Finally took time to make the tests pass. Please have a look. :grin:

[GeorchW]

I'd love to see this change land. Let me know if I can help.