mypy >=0.960 rejects macro_collector.py.
https://github.com/Mbed-TLS/mbedtls-framework/issues/50
We currently need mypy >=0.940, <0.960. Pick 0.942, which works, and is the
system version on Ubuntu 22.04.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>