Python-sane

- _sane.c:

- Fix compatibility with python3.6+

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- Removed support for Python 2

- _sane.c, sane.py:

- Add a progress parameter for the snap, arr_snap, scan and arr_scan methods.

- sane.py:

- Catch exceptions returned by self.device.snap
- Add __next__ to _SaneIterator for Python 3

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- _sane.c:

- Don't call sane_close on handle if sane_exit was called since

- Documentation.

- Fix example and documentation to reflect actual arr_snap array shape
- Fix various typos

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- _sane.c:

- Fix reading of 1bit scan data

- sane.py:

- Fix document feeder out of documents exception checking

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- sane.py:

- Fix array shape returned by sane.arr_snap()

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- _sane.c:

- Rewritten snap method, it now also works with backends which do not report
the number of lines in advance. Also, it now reads the data into a generic
memory buffer, removing the dependency on Pillow and numpy/numarray in the
C code. A PIL image is created via Image.frombuffer in sane.py now.
- General cleanup, fixing some Py_INCREF and Py_DECREF issues along the way.

- sane.py:

- Make arr_snap return a 3D numpy array, shaped (samples, width, height)
- Remove multipleOf support in arr_snap, this should be done by the caller
- Overall cleanup
- Expose the localOnly option to sane.get_devices()

- Documentation:

- New python-sphinx generated documentation, reworked example