Launchdarkly-server-sdk

Fixed:
- Changed `Files.new_data_source()` to use `yaml.safe_load()` instead of `yaml.load()` for YAML/JSON test data parsing. This disables `pyyaml` extended syntax features that could allow arbitrary code execution. ([136](https://github.com/launchdarkly/python-server-sdk/issues/136))

Fixed:
- Fixed an incompatibility with Python 3.3 due to an unpinned dependency on `expiringdict`.
- Fixed usages that caused a `SyntaxWarning` in Python 3.8. (Thanks, [bunchesofdonald](https://github.com/launchdarkly/python-server-sdk/pull/133)!)
- Updated CI scripts so a `SyntaxWarning` will always cause a build failure, and added a 3.8 build.

Added:
- The new `Config` parameter `http_proxy` allows you to specify a proxy server programmatically rather than by using environment variables. This may be helpful if you want the SDK to use a proxy, but do not want other Python code to use the proxy. (Thanks, [gangeli](https://github.com/launchdarkly/python-server-sdk/pull/130)!)

Fixed:
- Since version 6.1.0, the SDK was not respecting the standard `https_proxy` environment variable for specifying a proxy (because that variable is not used by `urllib3`). This has been fixed.
- In streaming mode, the SDK could fail to apply a feature flag update if it exceeded the LaunchDarkly service's maximum streaming message size; the service uses an alternate delivery mechanism in this case, which was broken in the SDK. This bug was also introduced in version 6.1.0.
- Fixed the generated documentation to exclude special members like `__dict__`.

Fixed:
- Fixed a bug in 6.10.0 that prevented analytics events from being generated for missing flags.

Added:
- Added support for upcoming LaunchDarkly experimentation features. See `LDClient.track()`.