Fix crash of GFile in python 3.7
Fix crash of GFile in python 3.7

This fix tries to address the issue raised in 27276 where
in Python 3.7, opening a zip file (of GFile) will results in
the error of
```
    bytes = self.zip.open(key)
  File "/usr/lib64/python3.7/zipfile.py", line 1480, in open
    self._fpclose, self._lock, lambda: self._writing)
  File "/usr/lib64/python3.7/zipfile.py", line 722, in __init__
    self.seekable = file.seekable
AttributeError: 'GFile' object has no attribute 'seekable'
```

The issue is that Python 3.7 adds seekable check:
https://github.com/python/cpython/commit/066df4fd454d6ff9be66e80b2a65995b10af174f

This fix adds `seekable()` and returns True, as GFile is indeed seekable.

This fix fixes 27276

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>

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