Fix HDF5Serializer test for h5py<2.9

[toslunar]

Chainer is tested with h5py>=2.5.
https://github.com/chainer/chainer-test/blob/b1b40421a2d7c73b86590575b19b82c801a12778/run_combination_test.py#L21

#8182 uses a feature of h5py>=2.9 (h5py/h5py#1061).

[niboshi]

How about using tempfile.NamedTemporaryFile?

import os
import tempfile


with tempfile.NamedTemporaryFile() as f:
    with open(f.name, 'w') as f2:
        f2.write('aa\n')
        f2.write('bb\n')

    with open(f.name, 'r') as f3:
        print(f3.readlines())

print(os.path.exists(f.name))

['aa\n', 'bb\n']
False

[toslunar]

Is the code guaranteed to run?

Whether the name can be used to open the file a second time, while the named temporary file is still open, varies across platforms (it can be so used on Unix; it cannot on Windows NT or later).

https://docs.python.org/3/library/tempfile.html#tempfile.NamedTemporaryFile

In any case, I'll add the comment to the code about which tempfile API should be used after the discussion is settled.

[niboshi]

@toslunar
OK, it seems NamedTemporaryFile cannot be used.

[niboshi]

LGTM
Jenkins, test this please

[chainer-ci]

Jenkins CI test (for commit 88041a8, target branch master) succeeded!

[chainer-ci]

@mergify[bot] This pull-request is marked as to-be-backported, but the corresponding backport PR could not be found. Could you check?