Upgrade mypy to 0.971, which is the last version that supports Python 3.6
(the oldest Python version that we currently run on the CI).
This fixes the error
```
framework/scripts/mbedtls_framework/outcome_analysis.py:119: error: Incompatible return value type (got "IO[Any]", expected "TextIO")
framework/scripts/mbedtls_framework/outcome_analysis.py:121: error: Incompatible return value type (got "IO[Any]", expected "TextIO")
```
As far as I can tell the fix is https://github.com/python/mypy/pull/9275
which was released in mypy 0.940.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
1c5a252729