This commit refactors the codebase to consistently use Python 3.6
f-strings for string concatenation and interpolation.

BREAKING CHANGE: This update increases the minimum required Python
version from 3.5 to 3.6. Python 3.5 reached end-of-life in September
2020 (~4 years ago) and has been dropped from most distributions, so the
impact on users should be minimal.

test_client.py uses pkg_resources. That's part of setuptools,
not Python. Per
https://docs.python.org/3.12/whatsnew/3.12.html#removed

 , it
was previously included by default in venv environments, but no
longer is from 3.12 onwards, so we need to explicitly require
setuptools to ensure it's available.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

This is a missing bit of the CI enablement, per
https://github.com/ymyzk/tox-gh-actions

 . I hope this will make
the Python 3.6 tests run properly.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

This drops the travis-ci config and adds config for running tests
via tox in Github Actions. It also updates the list of envs in
tox.ini for recent Python 3 versions.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

The `commands` line was accidentally(?) removed entirely in
57f716ed, so tox no longer actually ran any tests (only flake8).
This restores it, with the codestyle bit removed obviously. It
also fixes two lines where a tab was used instead of spaces.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

This fixes tests not being added to the source package.

I did this kind of on personal interest, since my emacs (using flake8)
marks every violation (meaning: line >79 chars) red, which is pretty
annoying.

I personally think 90 is a realistic guideline.

Remove python2.6 from
- Travis
- Tox
- PyPi tags
- documentation and readme

Also, Simplify an import clause.

This fixes #133

Should at least be compatible with Python 3.4 now.
Can't get Python 3.3 to work locally, but let's see with Travis.

- For some reason, 'pytest-cache' isn't found by setup.py if not
  specified explicitly in 'tests_require'
- Add tox.ini so tests can be run using tox if desired
- Update test to not be dependent on serialization