Add variable type annotations to engine.

[seisatsu]

Split from #148 and #151. I will bump our minimum Python version to 3.6 so we can do this.

[seisatsu]

Actually, this needs to be a future thing. My cutting edge distro doesn't even package 3.6 yet and there's no software that can make a Windows executable for 3.6. Too soon.

[seisatsu]

So, my distro now packages 3.6, and the program I use to package a Windows executable claims to support it as well. The future is now.

[pmer]

• Fedora: 3.6.2
• macOS Homebrew: 3.6.2
• VoidLinux: 3.6.2
• Windows: 3.6.2
• PyInstaller Support (untested): 3.6+