Close all open file handles in the test case
dulwich.tests.test_object_store.DiskObjectStoreTests:test_add_thin_pack
to avoid
WindowsError: [Error 32] The process cannot access the file because it is being used by another process: '...\\temp\\tmpdqhxlw\\pack\\pack-08ff98a2737425627a64fab11a4a5c53ed26bf9a.idx'
on Windows.
There should be DiskObjectStore.close() to make this practical!