Adding End-To-End Encryption to matrix-python-sdk

Goal

This project aims at fully implementing end-to-end encryption in matrix-python-sdk, following the official implementation guide and making use of the Python bindings already present in libolm.

Rationale

Currently, even though end-to-end encryption has landed in Matrix for over a year, only the three SDK upon which is built Riot on different platforms (JS, Android and iOS) ship with encryption. On desktop, this situation prevents writing clients, bots and bridges that can interact with encrypted rooms in any other language than javascript. Tackling this issue would benefit the matrix ecosystem by seeing more featureful projects emerge in a variety of languages (Python in this case). It also supports the Matrix long-term goal of having E2E encryption turned on by default in private rooms.