configs/qemu_arm_vexpress_tz_defconfig: optee needs host-python3 w/ modules

optee-os needs host-python-pycrypto build for python3. The only way we can
force building host-python modules for python3 is to select python3 package
for the target.

Since we want to avoid adding more host-python3-<modules>
(host-python-pycrypto host-python-pyelftools), select python3 package
even if it's not used.

This problem will be fixed as soon as python2 is removed.

Fixes:
File "scripts/pem_to_pub_c.py", line 24, in main
from Crypto.PublicKey import RSA
ImportError: No module named 'Crypto'

https://gitlab.com/buildroot.org/buildroot/-/jobs/456818689



Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Etienne Carriere <etienne.carriere@linaro.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>