Make sure to use a Python 3 pylint
Make sure to use a Python 3 pylint

On some systems, such as Ubuntu up to 19.04, `pylint` is for Python 2
and `pylint3` is for Python 3, so we should not use `pylint` even if
it's available.

Use the Python module instead of the trivial shell wrapper. This way
we can make sure to use the correct Python version.

Fix #3111

Signed-off-by: Gilles Peskine <f805f64266d288fc5467baa7be6cd0ff366f477b@arm.com>

Mbed-TLS/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls