CachingFileManager should not use __del__

[shoyer]

__del__ is sometimes called after modules have been deallocated, which results in errors printed to stderr when Python exits. This manifests itself in the following bug:
h5netcdf/h5netcdf#50

Per h5netcdf/h5netcdf#50 (comment), the right solution is probably to use weakref.finalize.

[stale]

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[shoyer]

This is still relevant