Installation failed in Python 3 with dependency error for futures

[jackyspy]

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• I have checked the superset logs for python stacktraces and included it here as text if any
• I have reproduced the issue with at least the latest released version of superset
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Superset version

0.22.1

Expected results

Actual results

When I use pip to install superset, it failed with error message "futures requires Python '>=2.6, <3' but the running Python is 3.5.4", which means futures package required. But futures is prepared for Python2, and it cannot be installed in Python3.

It seems to be an old issue with flower==0.9.1 which has already been resolved. So maybe we should change into flower==0.9.2 or flower without pinned version in setup.py.

Steps to reproduce

pip install superset==0.22.1

which fails as following

futures requires Python '>=2.6, <3' but the running Python is 3.5.4