Upgrade py_zipkin from 0.18.4 to 0.20.0 (#9777)
Upgrade py_zipkin from 0.18.4 to 0.20.0 (#9777)

https://github.com/Yelp/py_zipkin/blob/master/CHANGELOG.rst

Add create_http_headers helper
Add zipkin_span.add_annotation() method
Add autoinstrumentation for python Threads
Allow creating a copy of Tracer
Add extract_zipkin_attrs_from_headers() helper
Expose encoding.create_endpoint helper
Ensure tags are strings when using V2_JSON encoding
Add testing.MockTransportHandler module

[ci skip-jvm-tests]
pantsbuild/pants,pantsbuild/pants,benjyw/pants,jsirois/pants,jsirois/pants,benjyw/pants,jsirois/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,benjyw/pants,pantsbuild/pants