Add minimum supported python version is 3.7

[2xyo]

As date.fromisoformat is used and it's available from python 3.7 according to https://docs.python.org/3/library/datetime.html#datetime.date.fromisoformat

PR edited online and not tested 😅

[aeetos]

Thank you for submitting this - yes, you are correct, those methods in the analyzer require Python 3.7 or newer.

This library never specified a minimum Python version, so introducing it now is a big change that could create problems for other libraries that depend on it.

We need to make a decision on which version we will support, and plan for how that will impact dependent libraries. My initial thinking is that 3.6 is reasonable, but that will require some backporting to patch in a version of fromisoformat that will work in 3.6.

Due to these issues, I will close this PR without merging it in, but I have created a new issue #42 to track the problem. Feedback welcome on this approach.