This introduces djblets.util.filesystem.safe_join(), a port and

modernization of Django's internal django.utils._os.safe_join(). This

will join a series of paths and validate the result, making sure that

the resulting path is confined to the base path.
Our version also allows the result to be returned as a relative path,

and to use a specific pathing module (ntpath or posixpath). It also

provides type hints.
The main benefit is to avoid relying on an internal function that Django

may remove or substantially change at any time.

Unit tests pass.
Used this in some in-progress code.