Remove need for Python 2to3

All Python examples and tests have been written to be both Python 2 and Python 3
compatible, removing the need for 2to3 to run the examples or test-suite.

The 2to3 executable is not always available and even when available does not
always work, e.g. with pyenv. An alternative would be to use the lib2to3 Python
module instead, but this isn't available in some older versions of Python 3.

I had this problem on Ubuntu Bionic on Travis:

  checking Examples/python/callback
  pyenv: 2to3-3.8: command not found
  The `2to3-3.8' command exists in these Python versions:
    3.8
    3.8.1

Reference issues:
  https://github.com/pypa/virtualenv/issues/1399
  https://travis-ci.community/t/2to3-command-not-found-in-venv-in-bionic/4495

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