Fix is_optional(Literal[None])

[krzentner]

Without this, a field with a type like Literal[None, "a", "b"] may be considered required on some code paths. In particular, when specifying a default for an argument group. Note that this bug is difficult to work around because None | Literal["a", "b"] and Optional[Literal["a", "b"]] result in different (worse) parsing behavior.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.46%. Comparing base (b6c3346) to head (0ec5f81).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #323      +/-   ##
==========================================
+ Coverage   86.44%   86.46%   +0.01%     
==========================================
  Files          34       34              
  Lines        4531     4535       +4     
==========================================
+ Hits         3917     3921       +4     
  Misses        614      614              

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[lebrice]

Awesome! Thanks a lot @krzentner !