Parse custom scalar and enums in outputs

[leszekhanusz]

Adding parse_results functionality

If you add parse_results=True to the Client init,
or if you add parse_result=True to the execute or subscribe methods,
then gql will unserialize the result received from the backend with the scalars defined in the schema.

Note: because graphql-core PR 138 is not yet in a stable release, you might have issues with enums (parsed to None) if you get your schema from introspection.

New utilities method to update the schema

• update_schema_enum to update an Enum in the schema
• update_schema_scalar to update a Scalar in the schema

[codecov-commenter]

Codecov Report

Merging #256 (1e6ff7f) into master (d1ba78d) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master      #256    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           19        21     +2     
  Lines         1436      1650   +214     
==========================================
+ Hits          1436      1650   +214     

Impacted Files	Coverage Δ
gql/client.py	100.00% <100.00%> (ø)
gql/utilities/__init__.py	100.00% <100.00%> (ø)
gql/utilities/parse_result.py	100.00% <100.00%> (ø)
gql/utilities/serialize_variable_values.py	100.00% <100.00%> (ø)
gql/utilities/update_schema_enum.py	100.00% <100.00%> (ø)
gql/utilities/update_schema_scalars.py	100.00% <100.00%> (ø)

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