When creating a `gzip.GzipFile` using the named parameter `fileobj`
rather than filename, iterating over it is broken:
```
AttributeError: 'GzipFile' object has no attribute 'extrastart'
```

The short example below is demonstrating the problem:

## ---

import gzip, tempfile
_data = (b'@WXOVW:25:85', b'ATACGCGGCT'+b'GATCGTAGCG',
         b'+',
         b'@@))CCCCBB'+b'???ECCEECC')

data = gzip.zlib.compress(b'\n'.join(_data))
foo = tempfile.NamedTemporaryFile()
foo.write(data)
foo.flush()
foo.seek(0)

gzf = gzip.GzipFile(fileobj=foo)

# this will trigger the AttributeError
for x in gzf:
    print(x)