Remove long-deprecated Image.py functions

[hugovk]

Our current approach has been to deprecate for at least a year, and then remove in the next major version. I think this has been working fine (except for PILLOW_VERSION which didn't raise deprecation warnings; so we reinstated it with warnings).

Image.tostring and fromstring were deprecated in 2013, and Image.offset in 2001. They've raised either a DeprecationWarning, Exception or NotImplementedError for a long time. Let's remove them.

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im.tostring and fromstring

2000: added

Image.tostring, im.tostring and im.fromstring date back to at least PIL 1.1.1 (Oct 2000), probably earlier.

2013: DeprecationWarning

In Pillow 2.0.0 (March 2013), when adding support for Python 3, tobytes was added, and tostring was deprecated, raising DeprecationWarning: tostring() is deprecated. Please call tobytes() instead. And similarly with fromstring/frombytes (#35 & #71).

2015: Exception

In Pillow 3.0.0 (Oct 2015), the DeprecationWarning was replaced with an Exception (#1343).

2016: NotImplementedError

In Pillow 3.3.0 (July 2016), the Exception was replaced with a NotImplementedError (#1862).

NotImplementedError means this, and doesn't quite fit:

In user defined base classes, abstract methods should raise this exception when they require derived classes to override the method, or while the class is being developed to indicate that the real implementation still needs to be added.

Note: It should not be used to indicate that an operator or method is not meant to be supported at all – in that case either leave the operator / method undefined or, if a subclass, set it to None.

https://docs.python.org/3/library/exceptions.html#NotImplementedError

im.offset

2001: deprecated

2005: DeprecationWarning

2015: Exception

2001: NotImplementedError

Image.offset followed a similar path, except it was documented as deprecated in PIL 1.1.2 (May 2001), raised a DeprecationWarning in 1.1.5 (March 2005), then as above: Exception in Pillow 3.0.0 (Oct 2015), NotImplementedError in 3.3.0 (July 2016).