pypi: set SRC_URI with _prepend, not with +=

This did not cause problems in builds, but broke some devtool
workflows such as version upgrades or checking the latest version
from upstream. Tarballs should come first, not the patches.

(From OE-Core rev: 5cee50c25197102658e0689f635b2d567a375471)

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>