Modernize for 2020s

[originell]

Working on updating it all onto modern standards.

While this will be a major version, because we are dropping python 2 and python <3.5, there aren't any breaking changes. No behaviour has changed. This is just an update & cleanup release.

Overview:

• Pytest based testing
• Smaller fixes
• Documentation adjustments, and hopefully improvements
• Better deploy process to pypi
• Wheel based build process

TODO:

• [x] add many fancy project urls
• [x] Wand based engine ala https://docs.wand-py.org/en/latest/wand/image.html?highlight=watermark#wand.image.BaseImage.watermark
• [x] Convert based engine ala https://www.the-art-of-web.com/system/imagemagick-watermark/
• [x] libvips based engine ala https://github.com/libvips/php-vips/issues/40 and/or https://github.com/libvips/php-vips/blob/master/examples/watermark-text.php or maybe even https://github.com/libvips/pyvips/blob/master/examples/watermark_image.py
• [ ] Test case for composing jpg & pngs together - across all engines
• [x] Test case for templatetag to validate everything is passed in correctly
• [ ] Coverage reporting would be nice