v0.3.31

Fixup min python version (3.7 -> 3.8)

The correct minimum version is 3.8, as defined in the build-doc.yml;
the value in the pyproject.toml was residual from the first commits.
Also, 3.7 is on end-of-life.

Signed-off-by: Jorge Marques <jorge.marques@analog.com>