Py-zipkin

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- Support PRODUCER and CONSUMER spans

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- Add create_http_headers helper

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- Add zipkin_span.add_annotation() method
- Add autoinstrumentation for python Threads
- Allow creating a copy of Tracer
- Add extract_zipkin_attrs_from_headers() helper

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- Expose encoding.create_endpoint helper

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- Ensure tags are strings when using V2_JSON encoding

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- Add testing.MockTransportHandler module