Release v3.5.3 not update in version string

[jackbanditdude]

The release version for v3.5.3 was not incremented in the code, resulting in release version 3.5.3 still reading as "3.5.2".

VERSION = "3.5.2"

This should be corrected, especially as a recent update to the notifications for upcoming changes to Gigya note specifically that the Python SDK will be required to be v3.5.3 or higher: https://help.sap.com/whats-new/a487073f4f1e4777bbeff157b09f86d9?locale=en-US