Static type checking with mypy

This does introduce some not-so-nice things, like having to annotate each
`__init__` function with `-> None`. However, the benefits of having static
type checking in a complex bit of software like BAT outweigh the downsides.

blender_asset_tracer/trace/file_sequence.py

@@ -8,7 +8,7 @@ log = logging.getLogger(__name__)
 class DoesNotExist(OSError):
     """Indicates a path does not exist on the filesystem."""
 
-    def __init__(self, path: pathlib.Path):
+    def __init__(self, path: pathlib.Path) -> None:
         super().__init__(path)
         self.path = path