Bug#903399: Fix makes tag untestable

[Felix Lechner <felix.lechner@lease-up.com>]

The fix for this bug makes the tag 'old-python-version-field'
untestable. When the version for old-python equals ancient-python,
there is no way to reach the second block:

            if (versions_lte($v, $ancient)) {
                tag 'ancient-python-version-field', $field, $v;
            } elsif (versions_lte($v, $old)) {
                tag 'old-python-version-field', $field, $v;

The tags cannot be combined. They have different severities. I can
fake a test that swaps an actual 'ancient' tag for an 'old' tag, but
it would be a last resort. The test suite has almost full tag
coverage. Can we please find another solution, so that this tag is
testable, too?