c_client.py: Fix python-3 invalid except statement

Replace except statement with a PEP-3110 compliant one. This fixes a regression
introduced by c3deeaf714
https://bugs.freedesktop.org/show_bug.cgi?id=55690

Reviewed-by: Peter Harris <pharris@opentext.com>
Signed-off-by: Uli Schlachter <psychon@znc.in>

src/c_client.py

@@ -2908,7 +2908,7 @@ Refer to the README file in xcb/proto for more info.
 # Ensure the man subdirectory exists
 try:
     os.mkdir('man')
-except OSError, e:
+except OSError as e:
     if e.errno != errno.EEXIST:
         raise