enable tox to specify the Python version to use in sign-examples
The shebang of js2signed has been updated to Python 3, which means that
sign-examples now runs it using Python 3 by default. sign-examples is
used to setup the tox tests, which means that this change caused the
Python 2 tox environment to fail. As sign-examples is a shell wrapper
for the js2signed Python script, we can't simply invoke it using the
correct Python executable. So, instead, we add the SS_PYTHON
environment variable which allows us to specify which Python executable
to use (whilst still defaulting to using the default shebang if it isn't
set).
Filters were applied to versions before being applied to
items which meant if you were using --max option then
you often were not getting the expected output as the
version filter was filtering out the expected items.
This happens if there is a version greater than the version
of the item you expect despite the fact that the item
with the greater version never passing your supplied
filters.
Do not run flake8 during 'make test', remove 'trusty-flake8' tox env.
Previously 'make test' would run flake8. That meant that during
a package build a flake8 failure would cause package failure.
We do not really care about flake8 errors (primarily style) at that
point. Rather, upstream has picked versions of
flake8, pycodestyle, and pyflakes that it will ensure code is good with.
Along that same line, there is no need for the tox 'trusty-flake8'
environment which was present to catch those errors before a package
build on trusty.
The changes here mean that 'make test' won't run flake8. Note,
make can still be explicitly invoked with 'flake8' target.
