Fix S3 tests on Python 3

read_path() can read in binary, which the S3 tests don't support (simply
because they don't need it)...but it needs to be able to take the `mode`
argument anyway.

Signed-off-by: Ryan Gonzalez <ryan.gonzalez@collabora.com>

system/s3_lib.py

@@ -76,7 +76,10 @@ class S3Test(BaseTest):
         if self.check_path(path):
             raise Exception("path %s exists" % (path, ))
 
-    def read_file(self, path):
+    def read_file(self, path, mode=''):
+        # We don't support reading as binary here.
+        assert not mode
+
         if path.startswith("public/"):
             path = path[7:]