Use warnings.warn in tiff_file.py: get_values instead of stderr

[moi90]

I may have non-standard tiff files. Loading them tiff.TIFFfile writes an error to stderr, but the result is usable nevertheless.

get_values: incomplete info for type=7 ['7']: dtype=None, bytes=1

I expect pylibtiff to use warnings.warn instead, so that the user may handle such cases.

[moi90]

I created a pull request: #43

[moi90]

Are contributions like this unwanted?

[pearu]

No. Such contributions, especially pull requests, are highly appreciated. There was some time period where my github messaging was disabled (also in other projects) and so I missed many messages. Thanks again for your contribution!

It seems that pull request #43 got a conflict with previous Python 3 support merges..