Batteries included python package (#310)

Imports python bindings and associated sources incubated in
https://github.com/jerinphilip/lemonade to bergamot-translator. Adds
 a pybind11 dependency for python bindings.

Following the import, the python build is integrated into the existing 
CMake based build system here. There is a command-line application 
provided through python which provides the ability to fetch and prepare 
models from model-repositories (like browsermt/students or OPUS).

Wheels built for a few common operating systems are provided via GitHub
releases through automated actions configured to run at tagged semantic
versions and pushes to main.

The documentation for python is also integrated into our existing
documentation setup. Previous documentation GitHub action is now
configured to run behind python builds in Ubuntu 18.04 Python3.7,
in order to pick up the packaged as a wheel bergamot module and the
sphinx documentation using the python module.

Formatting checks of black, isort with profile black and a pytype type
checker is configured for the python component residing in this repository.

.github/workflows/coding-styles.yml

@@ -26,7 +26,7 @@ jobs:
 
         - name: Run clang-format
           run:
-              python3 run-clang-format.py --style file -r src wasm
+              python3 run-clang-format.py --style file -r src wasm bindings/python
 
 
         - name: Prepare build, compilation database etc.