FileStorage/py2 saves data with FS21 magic, while FileStorage/py3 with
FS30 magic:

https://github.com/zopefoundation/ZODB/blob/5.8.1/src/ZODB/_compat.py#L25-L77

Up till now FileStorage/go was accepting only FS21 and so with
py3-generated data it was leading to e.g. the following failure:

    === RUN   TestEmptyDB/py3_pickle3
        filestorage_test.go:90: testdata/py3_pickle3/empty.fs: invalid fs1 magic "FS30"

-> Fix it by accepting both py2 and py3 FileStorage magics.

We do accept them both without any other change because FileStorage
format, even when it comes with those two different magic, is really the
same.