Ranktier

Update Python versions to only cover supported Python version (except 3.7, which has EOL in 11 days).

* Added Player.name: This name is the one defined in your team settings - your Steam "alias" is Player.personaname.

* Moved error statements from STDOUT to STDERR
* README.md: Updated to reflect v1.4 changes
* setup.py: long_description is now read from README.md

* Python 2 support is dropped: Due to the fact that Python 2 will no longer be supported as of Jan 1st, 2020, ranktier has discontinued Python 2 support. This enables for more streamlined code, as support for both languages is no longer needed.
* Added ranktier.Player: Using the steamID3 found in the game client ("Friend ID") or on sites like [steamid.io](https://steamid.io), a rank can be retrieved from ranktier itself.
* setup.py: Read version number from ranktier/__init__.py

Added Immortal rank.

This mainly adds backend changes (moved function ranktier() to class Rank()). This meant to ease the future addition of a Rank().medal property which returns an image of the rank medal.