add __file__ to the globals available for tests loaded via DocFileSuite;
this is useful for locating supporting data files, just as it is in Python
modules
diff --git a/Lib/test/test_doctest3.txt b/Lib/test/test_doctest3.txt
new file mode 100644
index 0000000..54a96d5
--- /dev/null
+++ b/Lib/test/test_doctest3.txt
@@ -0,0 +1,5 @@
+
+Here we check that `__file__` is provided:
+
+  >>> type(__file__)
+  <type 'str'>