autotest: Add python-dateutil to external_packages.py.

Add the python-dateutil package
(https://dateutil.readthedocs.io/en/stable/), needed by the
tast_Runner server test to parse RFC3339 dates.

BUG=chromium:770439
TEST=ran build_externals.py

Change-Id: Ieb0a76ff381b6c6f0798a22cfc88e0daefbad9f8
Reviewed-on: https://chromium-review.googlesource.com/981179
Commit-Ready: Dan Erat <derat@chromium.org>
Tested-by: Dan Erat <derat@chromium.org>
Reviewed-by: Allen Li <ayatane@chromium.org>
diff --git a/utils/external_packages.py b/utils/external_packages.py
index 627f266..0f09160 100644
--- a/utils/external_packages.py
+++ b/utils/external_packages.py
@@ -1046,6 +1046,18 @@
             ExternalPackage._build_and_install_current_dir_setup_py)
 
 
+class DateutilPackage(ExternalPackage):
+    """python-dateutil package."""
+    version = '2.6.1'
+    local_filename = 'python-dateutil-%s.tar.gz' % version
+    urls = (_CHROMEOS_MIRROR + local_filename,)
+    hex_sum = 'db2ace298dee7e47fd720ed03eb790885347bf4e'
+
+    _build_and_install = ExternalPackage._build_and_install_from_package
+    _build_and_install_current_dir = (
+            ExternalPackage._build_and_install_current_dir_setup_py)
+
+
 class Pytz(ExternalPackage):
     """Pytz package."""
     version = '2016.10'