Represent serialized floats to approximately python float precision

[leszekhanusz]

Reproduce the changes made in graphql-core PR #164 so that the copy-pasted ast_from_value function in the dsl module stays in sync with the one in graphql-core.

[codecov-commenter]

Codecov Report

Merging #318 (22f4686) into master (1a1a2ee) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #318   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines         2135      2138    +3     
=========================================
+ Hits          2135      2138    +3     

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

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