MAINT: io: remove save_as_module
MAINT: io: move dumb_shelve and dumbdbm_patched into weave

Python's shelf module with dumbdbm backend doesn't seem to work for
weave for some reason, so stick with the patched version for now.

However, move it to a private module in weave, as it's no longer
needed in scipy.io.
DOC: remove mentions of save_as_module
DOC: release: update release notes re save_as_module removal