`DeprecationWarning` is not visible by default in the latest versions of
CPython, so this switches the deprecation warnings in setuptools and
pkg_resources over to custom classes derived from `Warning` instead.

Fixes issue github issue #159
Co-authored-by: Junhan Huang <robin.j.huang@gmail.com>
Co-authored-by: Marton Pono <marci93@gmail.com>