Python-SDK - 3.5.0

## [3.5.0] - July 9th, 2020

### New Features:
* Introduced 2 APIs to interact with feature variables:
* `get_feature_variable_json` allows you to get value for JSON variables related to a feature.
* `get_all_feature_variables` gets values for all variables under a feature.
* Added support for fetching authenticated datafiles. `AuthDatafilePollingConfigManager` is a new config manager that allows you to poll for a datafile belonging to a secure environment. You can create a client by setting the `datafile_access_token`.

### Bug Fixes:
* Fixed log messages for targeted rollouts evaluation. ([#268](