use static generation for json schema to python

[jrhizor]

Determine if

• https://github.com/pearmaster/json-schema-codegen
• https://github.com/microsoft/jschema-to-python
• or some other tool

allows us to use statically generated classes instead of dynamically built ones via python-jsonschema-objects

[jrhizor]

This should also fix the serialization problems from python-jsonschema-objects where certain key names result in serialization errors.

This should remove the temporary exclusion in singer_helpers.py

[jrhizor]

Right now https://github.com/koxudaxi/datamodel-code-generator/ is looking the most promising.