setup.py: set an upper bound on dependencies version

[JordanP]

This should make this library more robust in case of breaking changes in one of
its dependencies.

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[JordanP]

I signed it!

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[JordanP]

CI failed because it's now running with pytest version 5 released a few days ago and it introduced breaking changes... I guess that proves the point of setting an upper bound :)

[busunkim96]

Thank you @JordanP!