Gpiozero: Test fails using Python 3.7 on Debian Sid

[russel]

Using Python 3.6.6+ on Debian Sid, on GPIOZero e8bf5b500f9f2ea55d06869a86749b9c91691f68 having run "make develop", running py.test passes but delivers a number of deprecation warnings. For example:

…/Python_GPIOZero/tests/test_tools.py:173: DeprecationWarning: generator 'smoothed' raised StopIteration
  assert list(smoothed((), 5)) == []

On running with Python 3.7.0+ on Debian Sid, these warning become errors, StopIteration is raised

[russel]

This appears to be related to PEP 479 https://www.python.org/dev/peps/pep-0479/

[bennuttall]

Fixed in #670