Add support for Literal annotation

[dmontagu]

Change Summary

Properly handle attributes annotated with typing_extensions.Literal

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If you like this, let me know and I'll add documentation and finish the other bullets in the checklist. I'd also be happy to try implementing a workaround to eliminate the typing_extensions dependency (given some guidance about the desired outcome). I could also look into how to better integrate with the schema.

If you don't like this (e.g., because you think my implementation is awkward, or because you think this feature is just premature while Literal remains in typing_extensions rather than typing), no worries -- I'll just close the pull request and maybe come back to it at a later date. But the implementation seemed simple enough that I felt like it was worth sharing.

Related issue number

#561

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• HISTORY.rst has been updated
  • if this is the first change since a release, please add a new section
  • include the issue number or this pull request number #<number>
  • include your github username @<whomever>

[samuelcolvin]

I'm keen on the idea, just a few things to fix.

[codecov]

Codecov Report

Merging #582 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #582   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          15     15           
  Lines        2582   2613   +31     
  Branches      510    516    +6     
=====================================
+ Hits         2582   2613   +31

[samuelcolvin]

also need to:

• add this to docs
• update travis to run with and without typing_extensions, same as it currently does for ujson etc.

Otherwise this is looking great.

[samuelcolvin]

Looking good, just a few small things then please add to history and docs.

I'm confused about why the coverage appears to have dropped, maybe that is just codecov getting confused.

[lgtm-com]

This pull request introduces 2 alerts when merging c5ac4e7 into 461b852 - view on LGTM.com

new alerts:

• 2 for Module-level cyclic import

[lgtm-com]

This pull request introduces 1 alert when merging 9a3cd2d into 461b852 - view on LGTM.com

new alerts:

• 1 for Module-level cyclic import

[dmontagu]

@samuelcolvin thanks for merging this, I'll add docs soon

[samuelcolvin]

great thanks, somehow forgot about that.

[jasonkuhrt]

Great job all!