Allow graphql-core 3.2.6

[traylenator]

graphql-core changelog - https://github.com/graphql-python/graphql-core/releases

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (7fb869a) to head (0eeb7c9).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #545   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           38        38           
  Lines         2908      2856   -52     
=========================================
- Hits          2908      2856   -52     

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