Reflect that dateutil is depended upon by all of the package,

not just tests.

setup.py

@@ -5,7 +5,7 @@ shortdesc = 'iCalendar parser/generator'
 longdesc = open('README.rst').read()
 longdesc += open('CHANGES.rst').read()
 longdesc += open('LICENSE.rst').read()
-tests_require = ['unittest2', 'python-dateutil==1.5']
+tests_require = ['unittest2']
 
 setuptools.setup(
     name='icalendar',
@@ -30,6 +30,7 @@ setuptools.setup(
     zip_safe=False,
     install_requires=[
         'setuptools',
+        'python-dateutil==1.5',
         'pytz',
     ],
     extras_require={

tox.ini

@@ -6,7 +6,6 @@ deps =
     icalendar
     unittest2
     coverage
-    python-dateutil==1.5
 commands =
     coverage erase
     coverage run --source=icalendar --omit=*tests* {envbindir}/unit2 discover icalendar []