[issue3112] implement PEP 3134 exception reporting

[Antoine Pitrou]

Antoine Pitrou <pitrou at free.fr> added the comment:

Thinking about this, I realized that an exception can become its own
context if it is explicitly re-raised in its except handler (with "raise
variable", not bare "raise").
Not a critical bug, but it should be fixed in order to avoid delayed
garbage collection of such exceptions.

(as for the exception reporting code, it will have to detect recursivity)

>>> try: 1/0
... except Exception as e: raise e
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "<stdin>", line 1, in <module>
ZeroDivisionError: int division or modulo by zero
>>> e = sys.last_value
>>> e.__context__
ZeroDivisionError('int division or modulo by zero',)
>>> e.__context__ is e
True

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<http://bugs.python.org/issue3112>
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