Fix #3861 (calibre locks zip file when plugin not found)

src/calibre/customize/ui.py

@@ -3,6 +3,7 @@ __license__   = 'GPL v3'
 __copyright__ = '2008, Kovid Goyal <kovid at kovidgoyal.net>'
 
 import os, shutil, traceback, functools, sys, re
+from contextlib import closing
 
 from calibre.customize import Plugin, FileTypePlugin, MetadataReaderPlugin, \
                               MetadataWriterPlugin
@@ -51,7 +52,7 @@ def load_plugin(path_to_zip_file):
     #print 'Loading plugin from', path_to_zip_file
     if not os.access(path_to_zip_file, os.R_OK):
         raise PluginNotFound
-    zf = ZipFile(path_to_zip_file)
+    with closing(ZipFile(path_to_zip_file)) as zf:
         for name in zf.namelist():
             if name.lower().endswith('plugin.py'):
                 locals = {}