dateutil.parser import at top level, but python-dateutil not in requirements

[jmuhlich]

In simulator/base.py, dateutil.parser is used in the .load method for loading saved simulation results. That module is imported at the top level, but not listed in our requirements in setup.py. Under a "minimal" pysb install, importing pysb.simulator therefore fails. Our CI setups install pandas, which has python-dateutil as a dependency, so we never noticed the issue (this brings up a separate issue of how to fix CI to catch this in the future).

Since pysb generates this timestamp with isoformat(), we can parse it with datetime.fromisoformat() instead of the very general-purpose dateutil.parser.parse.