Remove use of deprecated method urlib.parse.splittag

[ijiraq]

Description

DeprecationWarning: urllib.parse.splittag() is deprecated as of 3.8, use urllib.parse.urlparse() instead

splittag is used in the following places

.//vos/vos/vos.py:    from urllib import splittag
.//vos/vos/vos.py:    splittag = six.moves.urllib.parse.splittag
.//vos/vos/vos.py:        (url, tag) = splittag(prop)
.//vos/vos/vos.py:        (url, prop_name) = splittag(prop)

Remove use of splittag.