Add tests capturing failure of depends.get_module_constant on Python 3.6. Ref #866.

pytest.ini

 [pytest]
-addopts=--doctest-modules --ignore release.py --ignore setuptools/lib2to3_ex.py --ignore tests/manual_test.py --ignore tests/test_pypi.py --ignore tests/shlib_test --doctest-glob=pkg_resources/api_tests.txt --ignore scripts/upload-old-releases-as-zip.py --ignore pavement.py
+addopts=--doctest-modules --ignore release.py --ignore setuptools/lib2to3_ex.py --ignore tests/manual_test.py --ignore tests/test_pypi.py --ignore tests/shlib_test --doctest-glob=pkg_resources/api_tests.txt --ignore scripts/upload-old-releases-as-zip.py --ignore pavement.py --ignore setuptools/tests/mod_with_constant.py
 norecursedirs=dist build *.egg setuptools/extern pkg_resources/extern .*
 flake8-ignore =
     setuptools/site-patch.py F821

setuptools/tests/mod_with_constant.py

+value = 'three, sir!'

setuptools/tests/test_depends.py

+import sys
+
+from setuptools import depends
+
+
+class TestGetModuleConstant:
+
+	def test_basic(self):
+		"""
+		Invoke get_module_constant on a module in
+		the test package.
+		"""
+		mod_name = 'setuptools.tests.mod_with_constant'
+		val = depends.get_module_constant(mod_name, 'value')
+		assert val == 'three, sir!'
+		assert 'setuptools.tests.mod_with_constant' not in sys.modules