Fix 'refleak' introduced by fnmatch cache purge tests.

This introduces a 'purge' function for the fnmatch module analogous
to the 'purge' function in the re module.
diff --git a/Doc/library/fnmatch.rst b/Doc/library/fnmatch.rst
index 7fa6148..ec78ed2 100644
--- a/Doc/library/fnmatch.rst
+++ b/Doc/library/fnmatch.rst
@@ -82,6 +82,13 @@
       <_sre.SRE_Match object at 0x...>
 
 
+.. function:: purge()
+
+   Clear the internal pattern cache.
+
+   .. versionadded:: 3.2
+
+
 .. seealso::
 
    Module :mod:`glob`