MLBEDSW-3367 Add mypy to pre-commit

Add mypy to pre-commit and clean up all reported errors.

Signed-off-by: Jonas Ohlsson <jonas.ohlsson@arm.com>
Change-Id: If7dc869f5fecdb0e2db40f14e7d9db21aa33df71
diff --git a/setup.py b/setup.py
index 488d296..031401e 100644
--- a/setup.py
+++ b/setup.py
@@ -98,6 +98,6 @@
     ],
     entry_points={"console_scripts": ["vela = ethosu.vela.vela:main"]},
     ext_modules=[mlw_module],
-    cmdclass={"build_ext": BuildExtension},
+    cmdclass={"build_ext": BuildExtension},  # type: ignore[dict-item]
     setup_requires=["numpy>=1.16.6,<=1.19.5", "setuptools_scm"],
 )