Found by Kirit Sankar Gupta.

* debug() calls sys.debughook()
* sys.debughook() by default calls pdb.set_trace()
* Add sys.__debughook__ to preserve the original.

Use Argument Clinic for the debug() docstring.

The biggest change is to use a setUp() that ensures a clean slate for the
tests, regardless of whether sys.breakpointhook or PYTHONBREAKPOINT is set.

* Improve the readability of the docstring.
* Add an assertion.
* Use Py_GETENV to obey -E