Dropping support for end-of-life Python versions

[papr]

This issue was created as a reference for MAINTENANCE.md (created in #155).

Currently, Python versions 2.7, 3.4, and 3.5 have reached their end of life. Python 3.6 will reach its end of life on 23 Dec 2016. Source.

Dropping support for these versions would allow the project to

• adapt the latest packaging best-practices (see "Building a distribution" in MAINTENANCE.md)
• adapt the latest type-annotation best-practices (as they had to be reverted in Restore test syntax compatibility with Python 2.7, 3.4, and 3.5 #152)

The decision for the process of no longer supporting specific Python versions can either be based on:

• Official Python support schedule as defined by https://devguide.python.org/#status-of-python-branches
• Based on usage (requires Pypi support #14 to be implemented)

[philrhc]

the project is no longer supporting python versions 2.7, 3.5, 3.6, 3.7
e434f79

[philrhc]

tests currently done using [3.8, 3.9],
i propose to expand this to [3.8, 3.9, 3.10, 3.11, 3.12, 3.13, 3.14]
is there anything else required before closing this ticket?
thanks