Modifies strings to all use the same quote where possible.
After running:
$ unify --in-place example.py
this code
a = "abc"
b = 'hello'
c = 'this \' is quote'
d = 'this " is another quote'
gets formatted into this
a = 'abc'
b = 'hello'
c= "this ' is quote"
d = 'this " is another quote'