7846: limit fnmatch pattern cache to _MAXCACHE=100 entries.

Patch by Andrew Clegg.
diff --git a/Misc/NEWS b/Misc/NEWS
index 729c010..b63c84c 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -470,6 +470,8 @@
 Library
 -------
 
+- Issue #7646: The fnmatch pattern cache no longer grows without bound.
+
 - Issue #9136: Fix 'dictionary changed size during iteration'
   RuntimeError produced when profiling the decimal module.  This was
   due to a dangerous iteration over 'locals()' in Context.__init__.