show @oneOf in introspection query

[leszekhanusz]

Allow to add isOneOf in the introspection query.

Needs graphql-core >= 3.3.0a7

Correspond to graphql-core PR #241

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7fb869a) to head (df84120).
⚠️ Report is 41 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master      #569    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           38        40     +2     
  Lines         2908      3240   +332     
==========================================
+ Hits          2908      3240   +332     

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