Launchdarkly-server-sdk

Added:
- Added py.typed file to indicate typing support. Thanks [phillipuniverse](https://github.com/launchdarkly/python-server-sdk/pull/166)

Fixed:
- Fixed invalid operator in key in TestData.
- Fixed bucketing logic to not treat boolean values as bucketable value types.

Added:
- `TestData`, in the new module `ldclient.integrations.test_data`, is a new way to inject feature flag data programmatically into the SDK for testing—either with fixed values for each flag, or with targets and/or rules that can return different values for different users. Unlike the file data source, this mechanism does not use any external resources, only the data that your test code has provided.

Added:
- CI builds now include a cross-platform test suite implemented in https://github.com/launchdarkly/sdk-test-harness. This covers many test cases that are also implemented in unit tests, but may be extended in the future to ensure consistent behavior across SDKs in other areas.

Fixed:
- The SDK no longer uses the deprecated method `threading.Condition.notifyAll()`. (Thanks, [jdmoldenhauer](https://github.com/launchdarkly/python-server-sdk/pull/162)!)
- A rule clause that uses a date operator should be considered a non-match, rather than an error, if either value is `None`.
- A rule clause that uses a semver operator should be considered a non-match, rather than an error, if either value is not a string.
- Rules targeting the `secondary` attribute will now reference the correct value.
- The `identify` method should not emit an event if the user key is an empty string.
- Do not include `prereqOf` field in event data if it is null. This is done to save on event transfer bandwidth.
- Data from `all_flags_state` was always including the flag's version even when it was unnecessary.
- Any base URIs set in `Config` will work consistently whether they have trailing slashes or not.
- When using `all_flags_state` to produce bootstrap data for the JavaScript SDK, the Python SDK was not returning the correct metadata for evaluations that involved an experiment. As a result, the analytics events produced by the JavaScript SDK did not correctly reflect experimentation results.
- Data from `all_flags_state` was always including the flag's version even when it was unnecessary.

Added:
- The SDK now supports evaluation of Big Segments. See: https://docs.launchdarkly.com/home/users/big-segments

Changed:
- Added CI testing for Python 3.10.

Fixed:
- In streaming mode, the SDK could sometimes fail to receive flag data from LaunchDarkly if the data contained characters that are not in the Basic Latin character set. The error was intermittent and would depend on unpredictable factors of speed and network behavior which could cause the first byte of a multi-byte UTF8 character to be processed before the rest of the bytes had arrived.
- Fixed some irregularities in the SSE parsing logic used for stream data. The SDK's CI tests now include a more thorough test suite for SSE behavior that is implemented in https://github.com/launchdarkly/sse-contract-tests, to ensure that it is consistent with other LaunchDarkly SDKs.

Added:
- The SDK now supports the ability to control the proportion of traffic allocation to an experiment. This works in conjunction with a new platform feature now available to early access customers.