Support for typing.Literal as an alternative to Enum in Python 3.8+.

[ghost]

Support for typing.Literal for creating choices, without actually having to create Enum types. Requires Python 3.8, referenced in #76. I ran tests again 3.6 without problems, so I think the version check if working OK.

[codecov]

Codecov Report

Merging #174 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #174   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          242       244    +2     
  Lines         4508      4545   +37     
=========================================
+ Hits          4508      4545   +37     

Impacted Files	Coverage Δ
docs_src/parameter_types/enum/tutorial003.py	100.00% <100.00%> (ø)
...test_parameter_types/test_enum/test_tutorial003.py	100.00% <100.00%> (ø)

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[carlthome]

Status on this PR @sretlawd? Changes LGTM but only skimmed them.

[blackary]

Friendly ping -- this would be a very nice feature to have.

[lukaszmoroz]

Hey it's been a year already, is there anything blocking this PR or is the project not maintained anymore? I'll allow myself ping @tiangolo

[ghost]

This looks really good. I'd like this!

[OskarZyg]

What happened with this PR?

[svlandeg]

What happened with this PR?

It looks like the original submitter removed their account, and Github then automatically closed this PR, as the edits don't seem to be available anymore :(

[svlandeg]

[Update]: This work has been continued in PR #429