ci: don't test on deprecated python versions

Remove Python 3.7 and 3.8 from the CI test matrix, as these versions are
deprecated / EOL. This enables usage of Python 3.9+ features in scripts,
such as str.removeprefix/str.removesuffix, and should cause any issue since
the scripts are always executed manually anyways.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>