convert: fix "stdlib import follows local import" problem in transport

Before this patch, import-checker reports error below for importing
subversion python binding libraries.

    stdlib import "svn.*" follows local import: mercurial

hgext/convert/transport.py

@@ -18,10 +18,6 b''
 # along with this program; if not, see <http://www.gnu.org/licenses/>.
 from __future__ import absolute_import
 
-from mercurial import (
-    util,
-)
-
 import svn.client
 import svn.core
 import svn.ra
@@ -29,6 +25,10 b' import svn.ra'
 Pool = svn.core.Pool
 SubversionException = svn.core.SubversionException
 
+from mercurial import (
+    util,
+)
+
 # Some older versions of the Python bindings need to be
 # explicitly initialized. But what we want to do probably
 # won't work worth a darn against those libraries anyway!