stager_test.py::StagerTest::test_with_main_session flaky on windows py3.6,3.7 (sickbayed)

[damccorm]

Frequently fails with:

TypeError: can't pickle zipimport.zipimporter objects

Example: https://github.com/apache/beam/pull/12505/checks?check_run_id=1183364761

Imported from Jira BEAM-10987. Original Jira may contain additional context.
Reported by: bhulette.