Implement `nonlocal` keyword in patchedast

[lieryan]

Describe the solution you'd like

Implement 'ast.Nonlocal` node in patchedast.py

Docs on how to implement this

• https://github.com/python-rope/rope/wiki/Supporting-new-syntax-for-refactoring

[julian-goettingen]

Your dedication to this library is awesome.