lint: fix mypy to v1.14.1

Set a fixed mypy version to ensure consistent builds. This is the most
recent version of mypy at the time of commit.

Signed-off-by: James Knight <git@jdknight.me>

Author: jdknight

tox.ini

@@ -82,7 +82,7 @@ commands =
 deps =
     {[testenv]deps}
     -r{toxinidir}/requirements_types.txt
-    mypy
+    mypy: mypy==1.14.1
 commands =
     mypy \
     --explicit-package-bases \