Test failure with Python 3.9 due to _dummy_thread removal

[stratakis]

When running the tests with Python 3.9 test_move_items fails with ModuleNotFound error. That module was removed with Python 3.9: https://bugs.python.org/issue37312

_dummy_thread should be replaced with _thread

[benjaminp]

Too bad Travis doesn't provide 3.9. I thought that would be what nightly is, but that seems to be the 3.8 branch.

[hroncok]

https://travis-ci.community/t/python-3-9-support/6703/4
travis-ci/travis-nightly-builder@41c2123

It seems that nightly is always just an alias to a harcoded 3.X-dev, not a nighly build from master :(

[hroncok]

Thank You!