change transports prototype using GraphQLRequest

[leszekhanusz]

Everywhere in the code we are sending the document, variable_values and operation_name as arguments of the methods.
Now that we have a GraphQLRequest class which encapsulate those properties, it makes sense to use it instead.

Simplifying transport execute and subscribe methods prototypes by using a GraphQLRequest object intead of document, variable_values and operation_name

Potential breaking changes:

• variable_values and operation_name are now keyword arguments only in client.py:
  you cannot do client.execute(query, params) anymore and need to do client.execute(query, variable_values=params) instead
• if a custom transport is used, it would need to change its execute and subscribe prototype to conform to the new methods.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (7fb869a) to head (5d234ed).
Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #551   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           38        40    +2     
  Lines         2908      3005   +97     
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+ Hits          2908      3005   +97     

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