Remove the `from __future__ import unicode_literals, print_function`.

**Describe the feature you'd like**
Because `byexample` runs on Python 3 only, these `__future__` imports (`unicode_literals` and `print_function`) are not needed anymore as they are the default.
Removing them will make the code a little cleaner.

When removing the imports from the examples/test, extra care should be taken to not break them or to hide some under-testing thing.