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.
Jordan Moldow