Fix inspect.getmodule to use a copy of sys.modules for iteration (#13487).

This fixes a regression compared to 2.x, where sys.modules.items()
returns a copy, as indicated by a comment in the source.  Diagnosis and
patch by Erik Tollerud.
diff --git a/Misc/ACKS b/Misc/ACKS
index 5d75404..4effde3 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -905,6 +905,7 @@
 Frank J. Tobin
 R Lindsay Todd
 Bennett Todd
+Erik Tollerud
 Matias Torchinsky
 Sandro Tosi
 Richard Townsend