Highlight typing hints in a different color than ordinary code.

@vpetrovykh đang làm issue này rồi.

Từ ngày 28/10/2019.

Typing hints are displayed in the same color as python variables and statements.
That looks somewhat confusing and makes it harder to see at once where is a typing hint, where a variable name is defined and where is an actual code.

E.g., the line below up to 'origin' is in the same color.
```
remote: pygit2.Remote = gitrepo.remotes['origin']
```