Python 3.9 support (removal of _dummy_thread)

[kevinvalk]

As described in https://bugs.python.org/issue37312, _dummy_thread and dummy_threading are removed in Python 3.9.

All packages using eight will fail to run on Python 3.9 (ModuleNotFoundError: No module named '_dummy_thread') because of importing _dummy_thread for rename as indicated

eight/eight/__init__.py

Line 161 in 586b081

'dummy_thread': '_dummy_thread'}

A fix in this package would be greatly appreciated!

[kislyuk]

Thank you for filing this issue and bringing this to my attention!

I have released a fix in v1.0.1 (the fix is to drop dummy_thread, since I doubt anyone uses it at this point). Please test and let me know if you encounter any other issues.