missing python3-dev requirements

[javiplx]

I've seen this while building an image for pinephone, not sure whether affects other builds.
Although it is indeed a requirements of https://gitlab.com/pine64-org/u-boot, the package appears in their gitlab-ci config, but as build_image_dist command continues despite the absence of Python.h I believe it should be fixed in this repo, either on documentation or in actual build commands