Pysstv

Added PD290 by clemibunge (24)

- entry point added by agsimmons (23)
- Pillow dependency version bumped to avoid security vulnerabilities (22) -- in case someone uses untrusted image sources

This version is no longer compatible with Python 2.x, Python 3.5 or later is required because of the Pillow library.

Fixed Python 3.9.1 compatibility, thanks to ebelliveau (20)

Fixed RGB modes silently depending on RGB images

Thanks to [Chris_J_Baird](https://twitter.com/Chris_J_Baird) on Twitter [reporting the issue](https://twitter.com/i/web/status/1149331458189737988):

> ColorSSTV.encode_line assuming its gets a tuple from `__getpixel__` had to make the `Image.open()` do a `convert('RGB')`

Also, the README format was explicitly defined as Markdown in `setup.py` [so that it now renders correctly on PyPI](https://pypi.org/help/#description-content-type) as well.

Added options for resizing (18)

- added PD modes (see 16)
- more readable usage text on Python 2.7+
- actualized readme (thx iDerp)