python3: make pydoc rdepend on python3-io

pydoc needs tempfile (provided by python3-io) to work. Add python3-io
to its RDEPENDS.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>