Tests break with coverage>4.2

[tomviner]

With coverage 4.3:

Successfully installed coverage
=========================================================================== test session starts ===========================================================================
platform linux2 -- Python 2.7.12, pytest-3.0.5, py-1.4.31, pluggy-0.4.0 -- /home/tom/.virtualenvs/tmp-70e6c02792b22275/bin/python2.7
cachedir: .cache
rootdir: /home/tom/dev/pytest-dev/pytest-ast-back-to-python, inifile: 
plugins: cov-2.4.0, ast-back-to-python-1.0.0
collected 3 items 

tests/test_ast_back_to_python.py::test_ast_as_python_on PASSED
tests/test_ast_back_to_python.py::test_ast_as_python_off PASSED
tests/test_ast_back_to_python.py::test_help_message PASSEDCoverage.py warning: Module pytest_ast_back_to_python.py was never imported.
Coverage.py warning: No data was collected.


=========================================================

See #3 for links to failing tests.

Using git bisect I pinned the failure down to this commit in the coverage github mirror:
nedbat/coveragepy@e7b7628

[tomviner]

Fixed in #3