scripts: requirements-extras: Request imgtool 1.9 for write-block-size > 8

On some targets (such as STM32U5) using write-block-size >8, imgtool
should be called using option '--align 16/32' which is only available
starting version 1.9.

Update requirement for imgtool to make it available to such platforms.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>