Improve tests to detect more setup.py issues

[Kami]

This pull request improves existing sdist tests so they also uncover issues which are related to setup.py depending on a 3rd party library such as the one inadvertently introduced in #27.

kami ~/w/stackstorm/st2sdk (git:master)$ tox -e py36-sdist
py36-sdist create: /home/kami/w/stackstorm/st2sdk/.tox/py36-sdist
py36-sdist installed: You are using pip version 9.0.1, however version 19.2.1 is available.,You should consider upgrading via the 'pip install --upgrade pip' command.,st2sdk==0.5.0
py36-sdist run-test-pre: PYTHONHASHSEED='3826356398'
py36-sdist run-test: commands[0] | rm -rf /tmp/py3-sdist
py36-sdist run-test: commands[1] | cp -r /home/kami/w/stackstorm/st2sdk /tmp/py3-sdist
py36-sdist run-test: commands[2] | python /tmp/py3-sdist/setup.py sdist --formats=zip
Traceback (most recent call last):
  File "py3-sdist/setup.py", line 17, in <module>
    import six
ModuleNotFoundError: No module named 'six'

Before, those checks would pass since six dependency was being installed in the virtual environment where that check runs.

[Kami]

@nmaludy Just a heads up.

I will also go ahead and publish v0.5.1 to PyPi once this is merged.

[Kami]

To confirm this change is also working on CI, I pushed a "dummy" issue - https://circleci.com/gh/StackStorm/st2sdk/326?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link, https://circleci.com/gh/StackStorm/st2sdk/325?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link.

[nmaludy]

:+1: