Fix Pytest4.x compatibility errors

This patch should fix such errors/warnings as:

raises / warns with a string as the second argument Deprecated since version 4.1.
pytest_funcarg__ prefix Removed in version 4.0.
getfuncargvalue
Metafunc.addcall Removed in version 4.0.

Fixes: https://github.com/pytest-dev/py/issues/209

But there are other not fixed warnings, for example:

testing/code/test_source.py:282
  testing/code/test_source.py:282: PytestCollectionWarning: yield tests were removed in pytest 4.0 - test_compilefuncs_and_path_sanity will be ignored
    def test_compilefuncs_and_path_sanity(self):

pytest-dev / py

Fix Pytest4.x compatibility errors #222