Adds 3.1.0 const feature

[spacether]

• writes out class that stores const values
• writes schema class data store that stores const value to name
• adds and uses SequenceNotStr to fix overload type checking for sequences that are not str or bytes

When working on this PR I found a bug in swagger-parser on parsing boolean + integer const values. I filed it here:
swagger-api/swagger-parser#1966

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