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.

README.md

@@ -23,3 +23,11 @@ There are two object types used to represent file paths. Those are strictly sepa
 When it is necessary to interpret a `bpathlib.BlendPath` as a real path instead of a sequence of
 bytes, BAT first attempts to decode it as UTF-8. If that fails, the local filesystem encoding is
 used. The latter is also no guarantee of correctness, though.
+
+
+## Type checking
+
+The code statically type-checked with [mypy](http://mypy-lang.org/).
+
+Mypy likes to see the return type of `__init__` methods explicitly declared as `None`. Until issue
+[#604](https://github.com/python/mypy/issues/604) is resolved, we just do this in our code too.