Always upgrade dev dependencies (e.g. Mock) to latest version
add a systest for bad cmdline options.
extract systest util call_process
Add new systest for Window
override docopt: exit value 1 on bad cmdline params
test_window should fail if there is no screenshot.png
The assert wasn't working, fixed now.

I think the parens were causing the first expression to the assert
(the condition) be a tuple, which always eval'ed True.
remove debug prints
New user story 'run', carries out behaviour formerly in 'main'.

Main now just instantiates Window adaptor to pass to user story 'run'.
Add options --screenshot, --exit. (not acted on yet)
Use correct Python3 metaclass declaration.

This prevents instantiation of WindowPyglet until it implements the
abstract methods.