⬆ Upgrade version required of Starlette from 0.19.1 to 0.20.4

[Kludex]

Runs against Starlette's master branch 0.20.0.

This version drops Python 3.6.

---

Edit by @tiangolo:

Upgrade Starlette version to 0.20.4, update imports for ExceptionMiddleware from the new location in Starlette.

Starlette is updated directly to 0.20.4 here as those 4 patch versions had several bug fixes, including security ones, this way it's possible to skip that, upgrade to the next feature version (from 0.19 to 0.20) and get all the recent bug fixes.

Support for Python 3.6 was just dropped in a previous PR. This PR also includes removing Pytest markers that were only needed for Python 3.6.

[codecov]

Codecov Report

Merging #4820 (339bd28) into master (1073062) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #4820   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          540       540           
  Lines        13947     13934   -13     
=========================================
- Hits         13947     13934   -13     

Impacted Files	Coverage Δ
..._sql_databases_peewee/test_sql_databases_peewee.py	100.00% <ø> (ø)
tests/test_union_inherited_body.py	100.00% <ø> (ø)
tests/utils.py	100.00% <ø> (ø)
fastapi/applications.py	100.00% <100.00%> (ø)
fastapi/security/api_key.py	100.00% <100.00%> (ø)

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[github-actions]

📝 Docs preview for commit e8cf7ff at: https://62654e2242ae77278b063442--fastapi.netlify.app

[github-actions]

📝 Docs preview for commit 8e3d62f at: https://62654f9dedbd5721c4ea0d8e--fastapi.netlify.app

[github-actions]

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[github-actions]

📝 Docs preview for commit 0878301 at: https://629311e9b6cde60a2ec687b8--fastapi.netlify.app

[MatthijsKok]

0.84.0 was just released. It drops support for Python 3.6, which this PR required.
Does that mean this branch can now be rebased and reviewed?
I can contribute to anything if needed, incrementally bumping starlette to its newest version will include some nice bugfixes.

[github-actions]

📝 Docs preview for commit 6f4b956 at: https://632318b625e2f334b644c7fe--fastapi.netlify.app

[github-actions]

📝 Docs preview for commit 339bd28 at: https://632319d64af9c0370f807924--fastapi.netlify.app

[tiangolo]

Thanks @Kludex! 🚀

I updated it a bit, raised the version to 0.20.4 and added a couple of tweaks.

Thanks for the discussion and help here everyone! ☕