BUG #8589: compression is "skipped" for pg_dump custom format when piped in python

The following bug has been logged on the website:

Bug reference:      8589
Logged by:          Tom Yan
Email address:      tom.ty89@gmail.com
PostgreSQL version: 9.2.4
Operating system:   Windows 7 x64
Description:

What I do is to pipe pg_dump -Fc to python's subprocess stdout.
The dump file I got is still usable with pg_restore, just it's not
compressed. It doesn't work even if I add -Z option manually.
And this is likely not a python's issue since the I can have compression
with -Z if I choose to export plain sql.