fix: Add six dependency to requirements/core.txt (#363)

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
------

-  #165

Author: The-inside-man

requirements/core.txt

@@ -3,4 +3,5 @@ pyrsistent==0.16.0
 requests>=2.21
 pyOpenSSL>=19.1.0
 cryptography>=2.8.0
-idna>=2.10
+idna>=2.10
+six>=1.12.0