Launchdarkly-server-sdk

Fixed:
- Under conditions where analytics events are being generated at an extremely high rate (for instance, if an application is evaluating a flag repeatedly in a tight loop on many threads), a thread could be blocked indefinitely within `variation` while waiting for the internal event processing logic to catch up with the backlog. The logic has been changed to drop events if necessary so threads will not be blocked (similar to how the SDK already drops events if the size of the event buffer is exceeded). If that happens, this warning message will be logged once: "Events are being produced faster than they can be processed; some events will be dropped". Under normal conditions this should never happen; this change is meant to avoid a concurrency bottleneck in applications that are already so busy that thread starvation is likely.

Fixed:
- Usages of `Logger.warn()` were causing deprecation warnings in some versions of Python. Changed these to `Logger.warning()`. ([125](https://github.com/launchdarkly/python-server-sdk/issues/125))

Changed:
- Changed the artifact name from `ldclient-py` to `launchdarkly-server-sdk`
- Changed repository references to use the new URL

There are no other changes in this release. Substituting `ldclient-py` version 6.9.1 with `launchdarkly-server-sdk` version 6.9.2 will not affect functionality.

Fixed:
- The `set_sdk_key` function was comparing the existing SDK key (if any) to the new one by identity (`is`) rather than equality (`==`). In Python, two strings that have the same characters may or may not be the same string instance; in the case where they were not, `set_sdk_key` would inappropriately reinitialize the client even though the SDK key had not really changed. (Thanks, [jpgimenez](https://github.com/launchdarkly/python-server-sdk/pull/121)!)
- Running the SDK unit tests is now simpler in that the database integrations can be skipped. See `CONTRIBUTING.md`.

Note on future releases:

The LaunchDarkly SDK repositories are being renamed for consistency. This repository is now `python-server-sdk` rather than `python-client`.

The package name will also change. In the 6.9.1 release, it is still `ldclient-py`; in all future releases, it will be `launchdarkly-server-sdk`. No further updates to the `ldclient-py` package will be published after this release.

Added:
- It is now possible to use the `with` statement on an LDClient object, so that `close()` will be called automatically when it goes out of scope.

Fixed:
- Calling `close()` on the client would cause an error if the configuration included `use_ldd=True`. ([118](https://github.com/launchdarkly/python-client/issues/118))

Fixed:
- Setting user attributes to non-string values when a string was expected would cause analytics events not to be processed. Also, in the case of the "secondary" attribute, this could cause evaluations to fail for a flag with a percentage rollout. The SDK will now convert attribute values to strings as needed. ([115](https://github.com/launchdarkly/python-client/issues/115))
- If `track` or `identify` is called without a user, the SDK now logs a warning, and does not send an analytics event to LaunchDarkly (since it would not be processed without a user).