Address code review

changelog/3191.feature.rst

@@ -10,7 +10,8 @@ When they should write::
 
 Because the ``assert_called_with`` method of mock objects already executes an assertion.
 
-This warning will not be issued when ``None`` is explicitly checked
-    assert none_returning_fun() is None
+This warning will not be issued when ``None`` is explicitly checked. An assertion like::
+
+    assert variable is None
 
 will not issue the warning

src/_pytest/assertion/rewrite.py

@@ -885,8 +885,7 @@ class AssertionRewriter(ast.NodeVisitor):
             warnings.warn_explicit(
                 PytestWarning('assertion the value None, Please use "assert is None"'),
                 category=None,
-                # filename=str(self.module_path),
-                filename=__file__
+                filename=str,
                 lineno=node.lineno,
             )
         """
@@ -905,7 +904,7 @@ warn_explicit(
     lineno={lineno},
 )
             """.format(
-                filename=str(module_path), lineno=lineno
+                filename=module_path.strpath, lineno=lineno
             )
         ).body
         return ast.If(val_is_none, send_warning, [])