setup.py doesn't specify requirement of datetuil

[stribb]

To reproduce:

1. In a project, run "pipenv install json-datetime".
2. Use it: "import jsondatetime as json"
3. Run the Python code

  File "/Users/andstrib/spt/secrets/.direnv/python-3.7.5/bin/store-account", line 7, in <module>
    exec(compile(f.read(), __file__, 'exec'))
  File "/Users/andstrib/spt/secrets/aws_secrets/bin/store-account", line 12, in <module>
    from aws_secrets import account, util
  File "/Users/andstrib/spt/secrets/aws_secrets/account.py", line 11, in <module>
    import jsondatetime as json
  File "/Users/andstrib/spt/secrets/.direnv/python-3.7.5/lib/python3.7/site-packages/jsondatetime/__init__.py", line 2, in <module>
    from .jsondatetime import loads, dumps
  File "/Users/andstrib/spt/secrets/.direnv/python-3.7.5/lib/python3.7/site-packages/jsondatetime/jsondatetime.py", line 3, in <module>
    import dateutil.parser
ModuleNotFoundError: No module named 'dateutil'```