Support PEP-415's Exception.__suppress_context__

PEP-415 states that `exception.__context__` should be suppressed
in traceback outputs, if `exception.__suppress_context__` is
`True`.

Now if a ``raise exception from None`` is caught by pytest,
pytest will no longer chain the context in the test report.

The algorithm in `FormattedExcinfo` now better matches the one
in `traceback.TracebackException`.

`Exception.__suppress_context__` is available in all of the
versions of Python 3 that are supported by pytest.

Fixes #2631.

_pytest/_code/code.py

@@ -679,7 +679,7 @@ class FormattedExcinfo(object):
                     e = e.__cause__
                     excinfo = ExceptionInfo((type(e), e, e.__traceback__)) if e.__traceback__ else None
                     descr = 'The above exception was the direct cause of the following exception:'
-                elif e.__context__ is not None:
+                elif (e.__context__ is not None and not e.__suppress_context__):
                     e = e.__context__
                     excinfo = ExceptionInfo((type(e), e, e.__traceback__)) if e.__traceback__ else None
                     descr = 'During handling of the above exception, another exception occurred:'