Add forgotten Attribute.evolve to stub

Fixes #752

changelog.d/752.change.rst

@@ -0,0 +1 @@
+Added forgotten ``attr.Attribute.evolve()`` to type stubs.

src/attr/__init__.pyi

@@ -83,6 +83,8 @@ class Attribute(Generic[_T]):
     kw_only: bool
     on_setattr: _OnSetAttrType
 
+    def evolve(self, **changes: Any) -> "Attribute": ...
+
 # NOTE: We had several choices for the annotation to use for type arg:
 # 1) Type[_T]
 #   - Pros: Handles simple cases correctly

tests/typing_example.py

@@ -242,6 +242,10 @@ class NGFrozen:
 
 ngf = NGFrozen(1)
 
+attr.fields(NGFrozen).x.evolve(eq=False)
+a = attr.fields(NGFrozen).x
+a.evolve(repr=False)
+
 
 @attr.s(collect_by_mro=True)
 class MRO: