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.

This commit updates some outdated syntax with the modern alternatives.
The changes were applied automatically using pyupgrade (v2.0.1) with the
following command:

```
pyupgrade --py3-plus **/*.py
```

These updates are purely syntactical and there should be no change in
functionality.

writeapi is being removed

Bug: T294397

Closes #207

Examples should adhere to PEP-8 guidelines, as these examples
not only document how to use correctly the client, but also
will serve as a first exposition to Python for some contributors.

Reference: https://www.python.org/dev/peps/pep-0008/

* make basic_edit.py and upload.py python-3 compatible

- Keep automated tests to be run with py.test in './tests/'
  and examples in './examples/'
- Rename test to adhere to 'Conventions for Python test discovery'
  (http://pytest.org/latest/goodpractises.html#test-discovery)