Highlight typing hints in a different color than ordinary code.

@vpetrovykh がすでに取り組んでいます。

2019年10月28日 から。

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']
```