Monkeypatch the 'setuptools.__file__' attribute in test setup to be absolute. Workaround for #852.

conftest.py

+import os
+
+
 pytest_plugins = 'setuptools.tests.fixtures'
 
+
 def pytest_addoption(parser):
     parser.addoption(
         "--package_name", action="append", default=[],
         help="list of package_name to pass to test functions",
     )
+
+
+def pytest_configure():
+    _issue_852_workaround()
+
+
+def _issue_852_workaround():
+    """
+    Patch 'setuptools.__file__' with an absolute path
+    for forward compatibility with Python 3.
+    Workaround for https://github.com/pypa/setuptools/issues/852
+    """
+    setuptools = __import__('setuptools')
+    setuptools.__file__ = os.path.abspath(setuptools.__file__)