Clear the test environment before each function run

See jupyter#231

jupyter_core/tests/test_paths.py

@@ -68,18 +68,15 @@
 prefer_env = patch.dict("os.environ", {"JUPYTER_PREFER_ENV_PATH": "True"})
 prefer_user = patch.dict("os.environ", {"JUPYTER_PREFER_ENV_PATH": "False"})
 
-resetenv = patch.dict(os.environ)
-
-
-def setup_module():
-    resetenv.start()
+def setup_function():
+    no_config_env.start()
     # For these tests, default to preferring the user-level over environment-level paths
     # Tests can override this preference using the prefer_env decorator/context manager
     os.environ["JUPYTER_PREFER_ENV_PATH"] = "no"
 
 
-def teardown_module():
-    resetenv.stop()
+def teardown_function():
+    no_config_env.stop()
 
 
 def realpath(path):
@@ -272,14 +269,14 @@ def test_runtime_dir_linux():
 
 def test_jupyter_path():
     system_path = ["system", "path"]
-    with no_config_env, patch.object(paths, "SYSTEM_JUPYTER_PATH", system_path):
+    with patch.object(paths, "SYSTEM_JUPYTER_PATH", system_path):
         path = jupyter_path()
         assert path[0] == jupyter_data_dir()
         assert path[-2:] == system_path
 
 
 def test_jupyter_path_user_site():
-    with no_config_env, patch.object(site, "ENABLE_USER_SITE", True):
+    with patch.object(site, "ENABLE_USER_SITE", True):
         path = jupyter_path()
 
         # deduplicated expected values
@@ -297,7 +294,7 @@ def test_jupyter_path_user_site():
 
 
 def test_jupyter_path_no_user_site():
-    with no_config_env, patch.object(site, "ENABLE_USER_SITE", False):
+    with patch.object(site, "ENABLE_USER_SITE", False):
         path = jupyter_path()
         assert path[0] == jupyter_data_dir()
         assert path[1] == paths.ENV_JUPYTER_PATH[0]