--import-mode: allow for "importlib"

Ref: pytest-dev/py#203

Author: blueyed

src/_pytest/python.py

@@ -101,7 +101,7 @@ def pytest_addoption(parser):
     group.addoption(
         "--import-mode",
         default="prepend",
-        choices=["prepend", "append"],
+        choices=["prepend", "append", "importlib"],
         dest="importmode",
         help="prepend/append to sys.path when importing test modules, "
         "default is to prepend.",