Run mypy on Travis
Run mypy on Travis

`tests/scripts/all.sh check_python_files` now runs mypy (in addition
to pylint) if it's available. So install mypy.

Install mypy 0.780, which is the earliest version that works on our
code at this time.

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

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