Specify minimum Python version, especially because fromisoformat() #42

The present library does not specify a minimum Python version in setup.py.

Some aspects of the analyzer module depend on datetime.fromisoformat() which is only available in Python 3.7 or earlier. Specifying 3.7 as the minimum version is the obvious fix, but may introduce problems for dependent libraries.

After testing and research, we likely need to target 3.6 as the minimum version, but that will require we implement some kind of backport for fromisoformat().

Until this is implemented, users who want to use date-specific functionality in the analyzer in Python versions earlier than 3.7 may want to consider this project (not yet tested by this library's authors):
https://github.com/movermeyer/backports.datetime_fromisoformat

Originally raised by @2xyo in #41