Use mypy config similar to other tket projects

Also add pytket as a mypy dependency

.pre-commit-config.yaml

@@ -35,6 +35,7 @@ repos:
       pass_filenames: false
       args: [.]
       additional_dependencies: [
-        pytest==7.4.2,
-        types-setuptools==68.2.0.0,
+        pytest,
+        types-setuptools,
+        pytket,
       ]

mypy.ini

@@ -28,7 +28,7 @@ disallow_untyped_decorators = true
 warn_redundant_casts = true
 warn_unused_ignores = true
 warn_return_any = true
-; warn_unreachable = true   # disabled because it can result in false positives
+warn_unreachable = true
 
 ## Miscellaneous strictness flags
 local_partial_types = true