support pytest 5.0+

[dirk-thomas]

This patch fixes #196 and addresses the failing CI due to the recent release of pytest 5.0. The change is backward compatible with older pytest versions.

• The first commit uses the new exit code constants.
• The second commit updates the use of the ExceptionInfo (which changes in str() on the pytest.raises context variable doesn't behave same as normal exception catch pytest-dev/pytest#5412)
• The third commit ignores the new deprecation warnings.