____________________________ test_regression_demo _____________________________
Traceback (most recent call last):
  File "t.py", line 2, in test_regression_demo
    testdir.makefile("tests.py", "# python file contents/n")
  File "site-packages/_pytest/pytester.py", line 1547, in makefile
    return py.path.local(str(self._pytester.makefile(ext, *args, **kwargs)))
  File "site-packages/_pytest/pytester.py", line 788, in makefile
    return self._makefile(ext, args, kwargs)
  File "site-packages/_pytest/pytester.py", line 756, in _makefile
    p = self.path.joinpath(basename).with_suffix(ext)
  File "lib/pathlib.py", line 889, in with_suffix
    raise ValueError("Invalid suffix %r" % (suffix))
ValueError: Invalid suffix 'tests.py'