devel/py-dateutil: Update to 2.7.3

• dateutil now uses setuptools_scm to derive its version which requires

a PyPI sdist or git checkout. USE_GITHUB uses tarballs, so switch to
CHEESEHOP accordingly.

Changelog:

https://github.com/dateutil/dateutil/blob/2.7.3/NEWS

PR: 230062
Submitted by: Radim Kolar <hsn sendmail cz>