Increased MAX_INT and MIN_INT to +(2^53 - 1) and -(2^53 - 1)

[jackton1]

Seems the comment uses the MAX_SAFE_INTEGER which should be 2^53 - 1 instead of 2^32 - 1.

graphql/graphql-spec#73

This might be platform dependent so switching to use sys.maxsize might be an alternative.

@syrusakbary

[erikwrede]

Using the custom BigInt Scalar is the preferred solution for this problem.
Everything else breaks spec:
https://spec.graphql.org/June2018/#sec-Int