tlsfuzzer.sh: Use python3

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
(Merged from openssl/openssl#23342)

(cherry picked from commit 15f479e)

Author: t8m

test/recipes/95-test_external_tlsfuzzer_data/tlsfuzzer.sh

@@ -42,7 +42,7 @@ OPENSSL_VERSION=$($CLI version | cut -f 2 -d ' ')
 TMPFILE="${PWD}/tls-fuzzer.$$.tmp"
 PSKFILE="${PWD}/tls-fuzzer.psk.$$.tmp"
 
-PYTHON=`which python`
+PYTHON=`which python3`
 PORT=4433
 
 echo "------------------------------------------------------------------"