setup.py: umask=022 for 'sdist', to avoid depending on environment

The new tarball-building buildslave had a bogus umask set, causing the 1.9.0
tarballs to be non-other-user-readable (go-rwx), which is a hassle for
packaging. (The umask was correct on the old buildslave, but it was moved to
a new host shortly before the release). This should make sure tarballs are
correct despite the host's setting.

Note to others: processes run under twistd get umask=077 unless you arrange
otherwise.

setup.py

@@ -311,7 +311,11 @@ class MySdist(sdist.sdist):
                 return fullname + "-SUMO"
             self.distribution.get_fullname = get_fullname
 
-        return sdist.sdist.make_distribution(self)
+        try:
+            old_mask = os.umask(int("022", 8))
+            return sdist.sdist.make_distribution(self)
+        finally:
+            os.umask(old_mask)
 
 
 setup_args = {}