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

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.