text: Title: Coverage is not measured with Python 3.7.0
  Body: I've noticed that `pytest-cov` doesn't report properly the coverage with Python 3.7.0. Here a simple repro:
  
  ### Directory structure
  ```
  |-- foo
  |   |-- __init__.py
  |   `-- bar.py
  `-- tests
      |-- __init__.py
      `-- test_baz.py
  ```
  
  ### Files content
  
  All `__init__.py` files are empty.
  #### `bar.py`
  ```
  def baz():
      print('baz')
  ```
  
  #### `test_baz.py`
  ```
  from foo import bar
  
  
  def test_foo():
      bar.baz()
  ```
  
  ### `pip freeze` for both Python 3.6.5 and 3.7.0 environments
  ```
  $ pip freeze
  atomicwrites==1.1.5
  attrs==18.1.0
  coverage==4.5.1
  more-itertools==4.2.0
  pluggy==0.6.0
  py==1.5.4
  pytest==3.6.3
  pytest-cov==2.5.1
  six==1.11.0
  ```
  
  ### Test results with Python 3.6.5 (correct result)
  ```
  $ py.test -p no:logging --strict --cov-report=term-missing --cov=foo tests
  =========================================== test session starts ============================================
  platform darwin -- Python 3.6.5, pytest-3.6.3, py-1.5.4, pluggy-0.6.0
  rootdir: /private/tmp/foo, inifile:
  plugins: cov-2.5.1
  collected 1 item
  
  tests/test_baz.py .                                                                                  [100%]
  
  ---------- coverage: platform darwin, python 3.6.5-final-0 -----------
  Name              Stmts   Miss  Cover   Missing
  -----------------------------------------------
  foo/__init__.py       0      0   100%
  foo/bar.py            2      0   100%
  -----------------------------------------------
  TOTAL                 2      0   100%
  
  
  ========================================= 1 passed in 0.03 seconds =========================================
  ```
  
  ### Test results with Python 3.7.0 (incorrect result)
  ```
  $ py.test -p no:logging --strict --cov-report=term-missing --cov=foo tests
  =========================================== test session starts ============================================
  platform darwin -- Python 3.7.0, pytest-3.6.3, py-1.5.4, pluggy-0.6.0
  rootdir: /private/tmp/foo, inifile:
  plugins: cov-2.5.1
  collected 1 item
  
  tests/test_baz.py .                                                                                  [100%]
  
  ---------- coverage: platform darwin, python 3.7.0-final-0 -----------
  Name              Stmts   Miss  Cover   Missing
  -----------------------------------------------
  foo/__init__.py       0      0   100%
  foo/bar.py            2      2     0%   1-2
  -----------------------------------------------
  TOTAL                 2      2     0%
  
  
  ========================================= 1 passed in 0.03 seconds =========================================
  ```
  