Automate upload to PyPI upon release

[chourroutm]

The latest version available on PyPI is https://pypi.org/project/stack-to-chunk/1.6/

The readthedocs is out of sync with PyPI.

[dstansby]

Sorry about this - I've just uploaded 1.10, and for version 2 (coming soon!) I'll automate it. Will leave this issue open to remind me to do the automation.