pylama not compatible with pytest 3.7.0

[bewing]

Pytest 3.7.0, recently released, does not work with the --pylama option, failing as follows:

.tox/py36/lib/python3.6/site-packages/_pytest/runner.py:201: in __init__
    self.result = func()
.tox/py36/lib/python3.6/site-packages/_pytest/runner.py:261: in <lambda>
    call = CallInfo(lambda: list(collector.collect()), "collect")
.tox/py36/lib/python3.6/site-packages/_pytest/main.py:475: in collect
    for x in self._collect(arg):
.tox/py36/lib/python3.6/site-packages/_pytest/main.py:519: in _collect
    for x in root._collectfile(pkginit):
E   AttributeError: 'PylamaItem' object has no attribute '_collectfile'

It appears to be related to this change:
pytest-dev/pytest@fedc785

Tested on both Python 2.7 and 3.6, and confirmed that tests collect and linting occurs on pytest==3.6.4, but fails with the above error on pytest==3.7.0

It looks like pytest is expecting pylama to implememt/override pytest.Package._collectfile(), but pylama.pytest.PylamaItem only inherits from pytest.File and pytest.Item. I'm not sure if the fix is also to inherit from pytest.Package, or if this is an issue in pytest where they should be checking if the plugin inherits from Package

[bewing]

Appears to be an upstream regression:

pytest-dev/pytest#3742