Downgrade mypy to a version that works with our code base

mypy >=0.960 rejects macro_collector.py.
#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>

scripts/ci.requirements.txt

@@ -7,9 +7,13 @@
 # 2.4.4 is the version in Ubuntu 20.04. It supports Python >=3.5.
 pylint == 2.4.4
 
-# Use the last version of mypy that is compatible with Python 3.6.
-# Newer versions should be ok too.
-mypy >= 0.971
+# Use a version of mypy that is compatible with our code base.
+# mypy <0.940 is known not to work: see commit
+#  :/Upgrade mypy to the last version supporting Python 3.6
+# mypy >=0.960 is known not to work:
+#   https://github.com/Mbed-TLS/mbedtls-framework/issues/50
+# mypy 0.942 is the version in Ubuntu 22.04.
+mypy == 0.942
 
 # At the time of writing, only needed for tests/scripts/audit-validity-dates.py.
 # It needs >=35.0.0 for correct operation, and that requires Python >=3.6,