Update sub_types when calling update_forward_refs()

[tiangolo]

Change Summary

Update sub_types when calling update_forward_refs()

update_forward_refs() currently updates a model's types by checking if they are ForwardRef.

This PR makes it also check sub-types, as would be needed for types like Union[ForwardRef('Node'), Leaf].

Related issue number

#404

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>

[codecov]

Codecov Report

Merging #464 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #464   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          14     14           
  Lines        2220   2225    +5     
  Branches      435    437    +2     
=====================================
+ Hits         2220   2225    +5

[tiangolo]

This is ready for your review @samuelcolvin.

Let me know if you want me to move the function update_field_forward_refs from main.py to utils.py.

[samuelcolvin]

otherwise LGTM.