allow "fake" filenames in findsource (closes #9284)

This allows findsource() to work in doctests.

A patch from Dirkjan Ochtman.
diff --git a/Misc/NEWS b/Misc/NEWS
index d3496b0..de1c531 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -22,6 +22,9 @@
 Library
 -------
 
+- Issue #9284: Allow inspect.findsource() to find the source of doctest
+  functions.
+
 - Issue #12009: Fixed regression in netrc file comment handling.
 
 - Issue #10694: zipfile now ignores garbage at the end of a zipfile.