graphene 3 compatibility

[AlexCLeduc]

Drop support for graphene 2, but keeps promises and our generator syntactic sugar working.

Still relying on an unpublished package, but everything seems to be working, including the graphiql debug toolbar

[codecov]

Codecov Report

Base: 74.21% // Head: 74.27% // Increases project coverage by +0.06% 🎉

Coverage data is based on head (1f4dc8b) compared to base (f1d155b).
Patch coverage: 85.71% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
+ Coverage   74.21%   74.27%   +0.06%     
==========================================
  Files          46       46              
  Lines        1629     1629              
==========================================
+ Hits         1209     1210       +1     
+ Misses        420      419       -1     

Impacted Files	Coverage Δ
zeus/changelog/graphql/util.py	73.46% <0.00%> (ø)
zeus/graphql/middleware.py	78.26% <ø> (+3.26%)	⬆️
zeus/graphql/internal_query_executor_base.py	79.54% <100.00%> (+0.47%)	⬆️

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