py3: use raw string for file open mode

Otherwise the module importer rewrites it to bytes on Python 3,
which angers Python 3.

Differential Revision: https://phab.mercurial-scm.org/D2137

mercurial/archival.py

@@ -218,7 +218,7 b' class zipit(object):'
                 dest.tell()
             except (AttributeError, IOError):
                 dest = tellable(dest)
-        self.z = zipfile.ZipFile(dest, 'w',
+        self.z = zipfile.ZipFile(dest, r'w',
                                  compress and zipfile.ZIP_DEFLATED or
                                  zipfile.ZIP_STORED)