Fix warning in make docs: target not found: graphql.execution.incremental_publisher.ExecutionResult

[leszekhanusz]

Fixing the following warnings which appeared when running make docs:

gql/client.py:docstring of gql.client.AsyncClientSession.execute:1: WARNING: py:class reference target not found: graphql.execution.incremental_publisher.ExecutionResult [ref.class]
gql/client.py:docstring of gql.client.AsyncClientSession.execute:1: WARNING: py:class reference target not found: graphql.execution.incremental_publisher.ExecutionResult [ref.class]
gql/client.py:docstring of gql.client.AsyncClientSession.execute_batch:1: WARNING: py:class reference target not found: graphql.execution.incremental_publisher.ExecutionResult [ref.class]
...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7fb869a) to head (c8fdbaa).
⚠️ Report is 45 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##            master      #579    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           38        40     +2     
  Lines         2908      3240   +332     
==========================================
+ Hits          2908      3240   +332     

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