scripts: use compatible release specifiers in requirements.txt

Using exact versions for our PyPI dependencies (i.e. requirements.txt
lines that look like "dependency==X.Y.Z") is rude since we're asking
users to install these dependencies outside of any virtual
environment. This causes conflicts with other packages which may
depend on the same things as us.

Use inclusive ordered comparison (>=) release specifiers instead:

https://www.python.org/dev/peps/pep-0440/#inclusive-ordered-comparison

If this causes problems for individual packages, we can refine them
using compatible release specifiers (~=):

https://www.python.org/dev/peps/pep-0440/#compatible-release



Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>