python3-package.mk: add check for dependencies in host-pip-requirements

python3-package.mk: add check for dependencies in host-pip-requirements

It often happens that we update a package to a new version (e.g. cffi) to a
newer version, but we forget to update the version for cffi in the
`lang/python/host-pip-requirements/cffi.txt` file.

This check adds a minimal check, so that when a build occurs for a Python
package, if there is a mention/listing of this package in
`lang/python/host-pip-requirements/` it will check that the versions match.

This way, when we update a package, we get a build failure and update the
host version as well.

This will omit packages (like Cython) that are not packaged for OpenWrt,
but are host-side dependencies only.
But until we find some mechanism to check for those, we will probably only
notice to update them when another build occurs (at the very least).

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>