Nested Zip not supported in Python < 3.7

[belldandyxtq]

Background
nested zip is zip inside a zip. For example we can have file in nested.zip in outside.zip as follow:

outside.zip
| - nested.zip
| - | - file

The following code is an example of how ChainerIO actually accesses such nested zipfile.

with zipfile.ZipFile("outside.zip", "rb") as outside_zip:
       with outside_zip.open("nested.zip", "rb") as inside_f:
              with zipfile.ZipFile(inside_f) as inside_zip:
                     with inside_f.open("file", "rb") as f:
                              f.read()

Both file_path and file object can be passed to create a zipfile.ZipFile.
Upon creating the zipfile.ZipFile, the zipfile module reads the header of the file to check whether the given file is a zip file. When checking, a "seek" is performed on the given file/file object, which requires the given file/file object to be seekable.

Problem

In Python<3.7, the ZipExtFile, which is the returned file object by the zipfile.open, i.e. the inside_f in the above code, is not seekable in any case. Which leads to failure on creation of ZipFile of the nested zip.

The problem was fixed in Python 3.7 as code