Drop support for unsupported Python versions (2.7 and 3.4-3.6)

[EwoutH]

I would want to open the discussion about dropping unsupported Python versions for the next release, and especially Python 2. By supporting only supported Python versions for new releases, new features can be adopted and no maintenance effort is poured into supporting old Python versions. Developers can contribute using modern Python syntax.

Practically, this would currently mean setting the minimum supported Python version to 3.7 for the next release, dropping support for Python 2.7, 3.4, 3.5 and 3.6.

For currently supported Python versions, see: https://endoflife.date/python

If consensus is reached about supporting Python 3.7+ for the next release:

• pyupgrade can be used with --py37-plus to update the codebase for the most part to modern Python standards
• In setup.py python_requires=">=3.7" should be added
• The Readme should be updated