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

This allows findsource() to work in doctests.

A patch from Dirkjan Ochtman.

Lib/test/test_inspect.py

@@ -295,6 +295,23 @@ class TestRetrievingSourceCode(GetSourceBase):
         del sys.modules[name]
         inspect.getmodule(compile('a=10','','single'))
 
+    def test_proceed_with_fake_filename(self):
+        '''doctest monkeypatches linecache to enable inspection'''
+        fn, source = '<test>', 'def x(): pass\n'
+        getlines = linecache.getlines
+        def monkey(filename, module_globals=None):
+            if filename == fn:
+                return source.splitlines(True)
+            else:
+                return getlines(filename, module_globals)
+        linecache.getlines = monkey
+        try:
+            ns = {}
+            exec compile(source, fn, 'single') in ns
+            inspect.getsource(ns["x"])
+        finally:
+            linecache.getlines = getlines
+
 class TestDecorators(GetSourceBase):
     fodderFile = mod2