Release 3.2.0

• setup.py: Remove test dependency doubles (#44)
• Do not throw error when no tracing headers are sent (#43)
• Stop testing with Python 3.5
• Provide meaningful error messages when failed to extract headers from carrier (#42) <Gabriel Falcão>
  • TextPropagator.extract raises SpanContextCorruptedException with
    more meaningful error messages.
  • Tracer.start_span validates type of references= preventing
    problems for users migrating code from opentracing==1.3.0.

Release 3.1.0

• Unpin OpenTracing dependency and change to a range (#40)
• Add support for Python 3.5+ (#39)
• Include 0,1 as acceptable sampled value (#37) <Dominik Janků>