configure: Require Python >= 3.9

We are using a couple of newer Python features which are only available
from 3.9 onwards.
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>

configure.ac

@@ -154,7 +154,7 @@ AM_CONDITIONAL(HAVE_SYSTEMD, [test "x$have_systemd" = "xyes"])
 # ------------------------------------------------------------------------------
 
 # Python
-AM_PATH_PYTHON([3.4])
+AM_PATH_PYTHON([3.9])
 PKG_CHECK_MODULES([PYTHON], [python-${PYTHON_VERSION}])
 
 # Perl