Modify gsettings-schema-convert to use python3. Also
add RDEPEND on python3-xml.
(From OE-Core rev: ce6e5da12fb004811270a37941094c9bd8d7e161)
Signed-off-by: Joe Slater <joe.slater@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>