mpatch: use absolute_import

While I was here, I removed the try..except around importing cStringIO
because cStringIO should always be importable on modern Python versions.
We already do an unconditional import in other files.

mercurial/pure/mpatch.py

@@ -5,11 +5,12 b''
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
+from __future__ import absolute_import
+
+import cStringIO
 import struct
-try:
-    from cStringIO import StringIO
-except ImportError:
-    from StringIO import StringIO
+
+StringIO = cStringIO.StringIO
 
 # This attempts to apply a series of patches in time proportional to
 # the total size of the patches, rather than patches * len(text). This

tests/test-check-py3-compat.t

@@ -122,7 +122,6 b''
   mercurial/manifest.py not using absolute_import
   mercurial/mdiff.py not using absolute_import
   mercurial/patch.py not using absolute_import
-  mercurial/pure/mpatch.py not using absolute_import
   mercurial/pure/osutil.py not using absolute_import
   mercurial/pure/parsers.py not using absolute_import
   mercurial/pvec.py not using absolute_import