Async generators always ensure that inner generator are closed properly

[leszekhanusz]

Previously, when using break inside async for session.susbcribe(..., the inner generator will not close properly because of the _generator variable reference which is still available from the session.

This PR will try to always ensure that if an async generator closes, the inner generators will also close.

This should ensure that a subscribe will close directly after a break if you don't keep any references of the generator

[codecov-commenter]

Codecov Report

Merging #230 (23ab7a2) into master (4e08f09) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #230   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        16           
  Lines         1064      1067    +3     
=========================================
+ Hits          1064      1067    +3     

Impacted Files	Coverage Δ
gql/client.py	100.00% <100.00%> (ø)

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