recipetool/create_buildsys_python: use importlib instead of imp

'imp' was deprecated in Python 3.4 and removed in 3.12. The
piece of importlib we use has been around since 3.3.

Signed-off-by: Chris Laplante <chris.laplante@agilent.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>