fix: Add six dependency to requirements/core.txt

[The-inside-man]

Summary

• Add six dependency as it was missing from requirements/core.txt

Note: This library is used throughout the core functionality of the SDK, however the build did not fail previously due to the six library being a transient dependency for jsonschema. Jsonschema is already in requirements/core.txt, however, we are now choosing to explicitly require the six dependency as we use this library explicitly in our SDK.

Test plan

• FSC

Issues

• Missing six package dependency from requirements.txt #165

[coveralls]

Coverage remained the same at 96.088% when pulling 0d6c2e6 on jbrown/add-six-dependancy into a1e31eb on master.

[msohailhussain]

lgtm. per our discussion need to add description in the summary.