cert_audit: Fix bug in check_cryptography_version

check_cryptography_version didn't provide helpful
message with Python < 3.6, because re.Match object
is not subscriptable.

Signed-off-by: Pengyu Lv <pengyu.lv@arm.com>
diff --git a/tests/scripts/audit-validity-dates.py b/tests/scripts/audit-validity-dates.py
index 81c69d3..35ea93c 100755
--- a/tests/scripts/audit-validity-dates.py
+++ b/tests/scripts/audit-validity-dates.py
@@ -45,7 +45,7 @@
 
 def check_cryptography_version():
     match = re.match(r'^[0-9]+', cryptography.__version__)
-    if match is None or int(match[0]) < 35:
+    if match is None or int(match.group(0)) < 35:
         raise Exception("audit-validity-dates requires cryptography >= 35.0.0"
                         + "({} is too old)".format(cryptography.__version__))