Remove use of six.reraise

[foolip]

Implementation of this for Python 3:

wpt/tools/third_party/six/six.py

Lines 697 to 706 in aa9b753

	def reraise(tp, value, tb=None):
	try:
	if value is None:
	value = tp()
	if value.__traceback__ is not tb:
	raise value.with_traceback(tb)
	raise value
	finally:
	value = None
	tb = None

Simplifying that step by step leads to the new code.

Part of #28776.

[foolip]

The apparent regressions for Firefox and Safari here are the same that I've seen a lot recently, see #28925. I'll admin merge this to bypass the failing stability checks, which timed out due to too many tests being affected.