Fix SF bug #976608, Unhelpful error message when mtime of a module is -1

Will backport.
diff --git a/Misc/NEWS b/Misc/NEWS
index 0826013..c78fa69 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -12,6 +12,8 @@
 Core and builtins
 -----------------
 
+- SF Bug #976608: fix SystemError when mtime of an imported file is -1.
+
 - SF Bug #887946: fix segfault when redirecting stdin from a directory.
   Provide a warning when a directory is passed on the command line.