test/recipes/95-test_external_pyca_data/cryptography.sh: use python3 command

Some distributions do not provide "python" command.  Change it
to python3 (which is hopefully more universally available) and provide
an ability to supply a different python command name, just in case.

Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Jakub Zelenka <jakub.zelenka@openssl.foundation>
Reviewed-by: Nikola Pajkovsky <nikolap@openssl.org>
Merge-date: Mon Sep  7 18:23:53 2026
Merged-from: https://github.com/openssl/openssl/pull/32668