Launchdarkly-server-sdk

Fixed:
- Removed an unused dependency on the `CacheControl` package.

Changed:
- The client now uses `urllib3` for HTTP requests, rather than the `requests` package. This change was made because `requests` has a dependency on an LGPL-licensed package, and some of our customers cannot use LGPL code. The networking behavior of the client should be unchanged.
- The client now treats most HTTP 4xx errors as unrecoverable: that is, after receiving such an error, it will not make any more HTTP requests for the lifetime of the client instance, in effect taking the client offline. This is because such errors indicate either a configuration problem (invalid SDK key) or a bug in the client, which will not resolve without a restart or an upgrade. This does not apply if the error is 400, 408, 429, or any 5xx error.
- During initialization, if the client receives any of the unrecoverable errors described above, `ldclient.get()` will return immediately; previously it would continue waiting until a timeout. The `is_initialized()` method will return false in this case.

Fixed:
- Fixed a bug introduced in v6.0.3 that caused the user cache for analytics events to never be cleared, also causing an `AttributeError` to appear in the log.

Removed:
- Removed a dependency on the `pylru` package, because it uses a GPL license.

Fixed:
- Fixed a bug that, in Python 3.x, could cause a timer thread to keep running after the client has been shut down. This bug also caused the message "TypeError: Event object is not callable" to be logged.
- Fixed the `Config` initializer to create a new instance of `InMemoryFeatureStore` if you omit the `feature_store` argument. Previously, all `Config` instances that were created with default parameters would share the same feature store instance.
- Clarified HTTP proxy setup instructions in the readme.

Fixed:
- Fixed a bug that caused an error message to be logged (`KeyError: 'default'`) when evaluating a prerequisite flag (and that also prevented an analytics event from being sent for that flag).
- When running in uWSGI, the client will no longer log an error message if the `enableThreads` option is absent, as long as the `threads` option has been set to a number greater than 1. ([84](https://github.com/launchdarkly/python-client/issues/84))

_This release was broken and has been removed._