Issue #18351: Fix various issues with
importlib._bootstrap._get_sourcefile().

Thanks to its only use by the C API, it was never properly tested
until now.

Thanks to Neal Norwitz for discovering the bug and Madison May for the patch.
diff --git a/Misc/ACKS b/Misc/ACKS
index cf64ed2..1392977 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -788,6 +788,7 @@
 Graham Matthews
 Dieter Maurer
 Daniel May
+Madison May
 Arnaud Mazin
 Rebecca McCreary
 Kirk McDonald