Install python-stdnum with Python 3.6 fails

[cedk]

Since 0e857fb python-stdnum requires at minimum Python 3.7 so the setup.py should reflect that with a proper python_requires otherwise pip tries to install the wrong version.

[arthurdejong]

Thanks for reporting this and providing the fix. It has been merged as 972b42d. I'll make a new release shortly with just this change.