Launchdarkly-server-sdk

Note that starting with this release, generated API documentation is available online at [https://launchdarkly-python-sdk.readthedocs.io](https://launchdarkly-python-sdk.readthedocs.io). This is published automatically from the documentation comments in the code.

Changed:
- The SDK no longer uses `jsonpickle`.
- The CI test suite for all supported Python versions, which previously only ran in Linux, has been extended to include Python 3.6 in Windows.

Fixed:
- Corrected and expanded many documentation comments. All public API methods (not including internal implementation details) are now documented.

Fixed:
- Fixed a _different_ packaging error that was still present in the 6.8.1 release, which made the package installable but caused imports to fail. The 6.8.1 release has been pulled from PyPI. We apologize for these recent errors, which were not detected prior to release because our prerelease testing was using the source code directly rather than installing it with `pip`. Our CI tests have been updated and should prevent this in the future.

Fixed:
- Fixed a packaging error that made the 6.8.0 release not installable. There are no other changes. The 6.8.0 release has been pulled from PyPI.

Added:
- It is now possible to use Consul as a persistent feature store, similar to the existing Redis and DynamoDB integrations. See `Consul` in `ldclient.integrations`, and the reference guide for ["Storing data"](https://docs.launchdarkly.com/sdk/features/storing-data#python).

Added:
- It is now possible to use DynamoDB as a persistent feature store, similar to the existing Redis integration. See `DynamoDB` in `ldclient.integrations`, and the reference guide to ["Storing data"](https://docs.launchdarkly.com/sdk/features/storing-data#python).
- The new class `CacheConfig` (in `ldclient.feature_store`) encapsulates all the parameters that control local caching in database feature stores. This takes the place of the `expiration` and `capacity` parameters that are in the deprecated `RedisFeatureStore` constructor; it can be used with DynamoDB and any other database integrations in the future, and if more caching options are added to `CacheConfig` they will be automatically supported in all of the feature stores.

Deprecated:
- The `RedisFeatureStore` constructor in `ldclient.redis_feature_store`. The recommended way to create a Redis feature store now is to use `Redis.new_feature_store` in `ldclient.integrations`.

Added:
- It is now possible to inject feature flags into the client from local JSON or YAML files, replacing the normal LaunchDarkly connection. This would typically be for testing purposes. See `file_data_source.py`.