Remove Python 2 support

Goals

• Remove Python 2 support
• Use f-strings (requires Python >= 3.6)

Prerequisites

• pyupgrade installed (the code examples below expect it to be on the search PATH.)

Steps

The following steps are necessary to remove Python 2 support from a package:

• setup.py
  • Update version number to next major version.
  • Remove Python 2 from the list of classifiers.
  • Remove six from the list of dependencies
  • Update python_requires to python_requires='>=3.6, <4',
  • Remove other things pointing to Python 2 or PyPy[2].
• setup.cfg
  • set universal = 0 in section [bdist_wheel]
• tox.ini
  • Remove py27 and pypy from `envlist
  • Remove Python 2 specific environments
• .travis.yml
  • Remove Python 2.7 and PyPy jobs.
• appveyor.yml
  • Remove Python 2 job(s) if existing.
• CHANGES.rst
  • Update the version number of the unreleased version
  • Add an entry: Drop support for Python 2.
• Remove Python 2 support code
  • Run find src -name "*.py" -exec pyupgrade --py3-plus {} \;
    • Update the code to Python 3 e. g. by removing (some of) six usage.
  • Run grep -rn six src
    • Remove or change all the usages of six
  • Run the tests: tox + fix problems
  • Commit the current status to a branch.
  • Run egrep -rn "2.7|sys.version|PY2|PY3|Py2|Py3|Python 2|Python 3|__unicode__|ImportError" src
    • Fix branching depending on Python version etc.
  • Run find src -name "*.py" -exec pyupgrade --py36-plus {} \;
    • Update code to more Python 3 only goodies.
• Create pull request from your changes.
• Create a new branch from master before your changes to be able to back port fixes later on easily to the version which still supports Python 2.

Open issues

• document needed changes if a package is tests on Appveyor