BUG: AttributeError: 'function' object has no attribute 'im_func' on Py3.

Needs review. 
Fixes one test error on Python 3:
```
Tests\test_file_webp_metadata.py:22: exif = image._getexif() failed:
Traceback (most recent call last):
  File "Tests\test_file_webp_metadata.py", line 22, in test_read_exif_metadata
    exif = image._getexif()
  File "PIL\WebPImagePlugin.py", line 43, in _getexif
    return JpegImageFile._getexif.im_func(self)
AttributeError: 'function' object has no attribute 'im_func'
```