Wm-sampledata

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- Nothing changed yet.

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- setting env variable CI=true allows to disable image download
(useful for running tests that generate sampledata)

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- use pypi compatible email-address in setup.py

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- use python-requests for downloading images

- breaking: return image data as string (no StringIO buffers)

- `utils.get_placeholder_image` allows to obtain images from
https://placeholder.com/
[fRiSi]

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- Fix the user agent switcher to be compatible with Mac/Windows.
[pcdummy]

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- Add a user-agent to the image downloader.
[pcdummy]