Drop Python 2.7 support

[stefankoegl]

In #47 the question has come up whether to drop support for Python 2.7.

I don't have any opinion on that yet, but I'd like to open the discussion.

A related issue is open for jsonpatch at stefankoegl/python-json-patch#140.

Input

The charts on https://pypistats.org/packages/jsonpointer show download stats based on Python versions.

Sunset date for the Python project itself was Jan 1, 2020: https://www.python.org/doc/sunset-python-2/

See https://python3statement.org/ for projects that have dropped support for Python 2 by/before the sunset date.