py3: make second argument of fdopen() a str

# skip-blame as just r'' prefixes

Differential Revision: https://phab.mercurial-scm.org/D5260

hgext/remotefilelog/basepack.py

@@ -373,8 +373,8 b' class mutablebasepack(versionmixin):'
             suffix=self.PACKSUFFIX + '-tmp')
         self.idxfp, self.idxpath = opener.mkstemp(
             suffix=self.INDEXSUFFIX + '-tmp')
-        self.packfp = os.fdopen(self.packfp, 'w+')
-        self.idxfp = os.fdopen(self.idxfp, 'w+')
+        self.packfp = os.fdopen(self.packfp, r'w+')
+        self.idxfp = os.fdopen(self.idxfp, r'w+')
         self.sha = hashlib.sha1()
         self._closed = False