Adapt tests helper to Coverage 4.0 (ref #4175).

helper.py

@@ -303,15 +303,24 @@ def _save_coverage(cov, result, rootdir, testing_path):
     # long as 2to3 is needed). Therefore we only do this fix for
     # Python 2.x.
     if six.PY2:
-        d = cov.data
-        cov._harvest_data()
-        for key in d.lines.keys():
+        try:
+            # Coverage 4.0: _harvest_data has been renamed to get_data, the
+            # lines dict is private
+            cov.get_data()
+        except AttributeError:
+            # Coverage < 4.0
+            cov._harvest_data()
+            lines = cov.data.lines
+        else:
+            lines = cov.data._lines
+
+        for key in lines.keys():
             new_path = os.path.relpath(
                 os.path.realpath(key),
                 os.path.realpath(testing_path))
             new_path = os.path.abspath(
                 os.path.join(rootdir, new_path))
-            d.lines[new_path] = d.lines.pop(key)
+            lines[new_path] = lines.pop(key)
 
     color_print('Saving coverage data in .coverage...', 'green')
     cov.save()