Fix response_model type to allow List[Model]

[tiangolo]

🐛 Fix response_model type to allow List[Model].

Mainly to fix mypy for users.

[codecov]

Codecov Report

Merging #266 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #266   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files         190    190           
  Lines        4649   4649           
=====================================
  Hits         4649   4649

Impacted Files	Coverage Δ
fastapi/routing.py	100% <ø> (ø)	⬆️
fastapi/applications.py	100% <ø> (ø)	⬆️

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