Broken Python 3.5 without matplotlib

[mwtoews]

It seems that the flopy-3.3.1 release doesn't always work with Python 3.5, e.g.:

    import flopy
../../../virtualenv/python3.5.6/lib/python3.5/site-packages/flopy/__init__.py:26: in <module>
    from . import modflow
../../../virtualenv/python3.5.6/lib/python3.5/site-packages/flopy/modflow/__init__.py:1: in <module>
    from .mf import Modflow
../../../virtualenv/python3.5.6/lib/python3.5/site-packages/flopy/modflow/mf.py:10: in <module>
    from ..mbase import BaseModel
../../../virtualenv/python3.5.6/lib/python3.5/site-packages/flopy/mbase.py:22: in <module>
    from flopy import utils, discretization
../../../virtualenv/python3.5.6/lib/python3.5/site-packages/flopy/utils/__init__.py:49: in <module>
    from .gridintersect import GridIntersect, ModflowGridIndices
../../../virtualenv/python3.5.6/lib/python3.5/site-packages/flopy/utils/gridintersect.py:4: in <module>
    except ModuleNotFoundError:
E   NameError: name 'ModuleNotFoundError' is not defined

however this release works with Python 3.5 if matplotlib is installed, which is why the Travis CI tests run without error.

Two possible fixes:

• Change ModuleNotFoundError to ImportError for Python 3.5 compatibility
• Bump the minimum Python version to 3.6