Clone of @virtuald's PR #2112 with minor enhancements. (#3215)

* Add py::raise_from to enable chaining exceptions on Python 3.3+

* Use 'raise from' in initialization

* Documenting the exact base version of _PyErr_FormatVFromCause, adding back `assert`s.

Co-authored-by: Dustin Spicuzza <dustin@virtualroadside.com>