Add type hints to 'parsing/' package

[lavovaLampa]

Initial batch of type hints. Also refactored token kinds into an Enum, as Python 3.6 is no longer supported.

Currently depends on some types provided by package 'typing-extensions', in case this new dependency is unwelcome, I can remove dependent type hints.

[umarcor]

CI is failing because the required package is not installed. You can specify it in https://github.com/VUnit/vunit/blob/master/pyproject.toml#L64-L80, since all the tests are executed through tox.

[lavovaLampa]

While setuptools is trying to determine dependencies, it has to run the setup.py script. It then tries to import other modules, running into an import from typing_extensions. AFAIK, setuptools builds packages in isolated environment, meaning this dependency is not present at the time of dependency resolution (makes sense).

[lavovaLampa]

Project metadata could be alternatively moved into pyproject.toml. In any case this should solve the issue of dependency definitions during build.

[umarcor]

@lavovaLampa could you please make 7cfdaec a separated PR (which we can merge before this)?

[lavovaLampa]

Opened #942