Python 2.7 does not install typing package

[todofixthis]

When installing PyOTA on a Python 2.7 virtualenv, the typing package does not get installed:

(test) [499][~] python --version
Python 2.7.13
(test) [500][~] pip install --pre pyota
...
Installing collected packages: six, python-dateutil, regex, pytz, filters, requests, pyota
Successfully installed filters-1.1.4 pyota-1.0.0b3 python-dateutil-2.6.0 pytz-2016.10 regex-2016.12.27 requests-2.12.4 six-1.10.0

[todofixthis]

Fixed.

• Wheels do not include setup.py, so all of that fancy version-detection logic is lost.
• There is a way to specify environment markers in project dependencies, but it requires a fairly recent version of setuptools. For compatibility, we will just install typing on all environments, even if not needed.

We might be able to accomplish Python-version-specific dependencies using requirements files; it's worth looking into at some point when more bandwidth frees up (: