This lets us cut out the line which hard-codes how deeply nested in
the tree the `run-mypy` script is, making it simpler to borrow these
scripts in other projects.
Previously lister.py used to check whether the exclude path is a
substring of a path being considered. So it would fail when the
exclude path is an absolute path or uses '..' or '.'.
Make module tools/lister.py which lists all files in a directory
tracked by git. This is done because lister.py will be used by other
scripts in the future which have to introspect files in the repository,
like linters, static code checkers, etc.