v0.7.8 includes a missing import in common/transports/sync.py

[cranti]

Describe your environment.

• Python 3.7.7
• Platform: Darwin-19.2.0-x86_64-i386-64bit
• v0.7.8

Steps to reproduce.
The source in PyPI for v0.7.8 is missing an import line in common/transports/sync.py:

from opencensus.trace import execution_context

It is present on master.

What is the actual behavior?
A NameError when end_span is called:

  File "/Users/cranti/.venv/lib/python3.7/site-packages/opencensus/trace/span.py", line 392, in __exit__
    self.context_tracer.end_span()
  File "/Users/cranti/.venv/lib/python3.7/site-packages/opencensus/trace/tracers/context_tracer.py", line 123, in end_span
    self.exporter.export(span_datas)
  File "/Users/cranti/.venv/lib/python3.7/site-packages/opencensus/trace/logging_exporter.py", line 86, in export
    self.transport.export(span_datas)
  File "/Users/cranti/.venv/lib/python3.7/site-packages/opencensus/common/transports/sync.py", line 24, in export
    execution_context.set_is_exporter(True)
NameError: name 'execution_context' is not defined

[lzchen]

Should be fixed with v0.7.9