Bump version to 0.16.3
pithcat: Fix initialization of Pithos backend

Make pithcat initialize the Pithos backend (ModularBackend) using
keywords instead of positional arguments.

Also, do not use the 'None' argument to specify that the default value
of Pithos backend should be used. The behavior of Pithos backend has
changed, and passing 'None' will make the backend use the 'None' value.
pithcat: Set default umask to 007
Update ChangeLog and version files for 0.16.3
Merge branch 'hotfix-0.16.3'