Upgrade to Python 3.7+

[EwoutH]

This PR upgrades the code base to Python 3.7, including:

• Requiring Python 3.7 in setup.py
• Upgrades to Python 3.7+ syntax using Pyupgrade and flynt
• Updating the Readme
• Updating the packaging CI See each individual commit for details.

Python 3.6 support has been dropped almost a year ago and Python 2.7 is end-of-life for almost 3 years now.

These changes affect only future releases, older releases like 2.6.0 can still be used with the old Python versions.

Closes #552.

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[stumitchell]

Yeah we have kept 2.7 support for a long time. But it is probably holding us back now. Can we confirm that python 2.7 users would still be able to find and install pulp through pip etc. i.e. it will point them to a previous version. I think this should also be a Major Version number increment @pchtsp your thoughts?

[EwoutH]

Can we confirm that python 2.7 users would still be able to find and install pulp through pip etc. i.e. it will point them to a previous version.

Yes. pip searches for the python_requires tag I added to setup.py. Previous releases didn’t have that, so pip will try to install 2.6.0 on all previous Python versions. But all new releases will only be installed on Python 3.7 and higher.

See https://packaging.python.org/en/latest/guides/distributing-packages-using-setuptools/#python-requires

[pchtsp]

sorry for the delay, I've been quite busy. I agree we can drop python 2.7. I'm going to release a last version with the current master. We can then get all the other PRs that assume python >= 2.7, and we can change to version 3.0, while updating docs.