Enable support for PEP 526 annotated assignments (#84)

Without this change, code with annotated assignments is crashing pyflakes with
an attribute error (`object has no attribute 'ANNASSIGN'`).

Test plan: new tests introduced conforming to behavior described in the PEP.

Author: ambv

pyflakes/checker.py

@@ -1335,3 +1335,21 @@ def EXCEPTHANDLER(self, node):
                 del self.scope[node.name]
             except KeyError:
                 pass
+
+    def ANNASSIGN(self, node):
+        """
+        Annotated assignments don't have annotations evaluated on function
+        scope, hence the custom implementation.
+
+        See: PEP 526.
+        """
+        if node.value:
+            # Only bind the *targets* if the assignment has a value.
+            # Otherwise it's not really ast.Store and shouldn't silence
+            # UndefinedLocal warnings.
+            self.handleNode(node.target, node)
+        if not isinstance(self.scope, FunctionScope):
+            self.handleNode(node.annotation, node)
+        if node.value:
+            # If the assignment has value, handle the *value* now.
+            self.handleNode(node.value, node)

pyflakes/test/test_other.py

@@ -1808,3 +1808,64 @@ def test_formatstring(self):
         mom = 'mom'
         f'{hi} {mom}'
         ''')
+
+    @skipIf(version_info < (3, 6), 'new in Python 3.6')
+    def test_variable_annotations(self):
+        self.flakes('''
+        name: str
+        age: int
+        ''')
+        self.flakes('''
+        name: str = 'Bob'
+        age: int = 18
+        ''')
+        self.flakes('''
+        class C:
+            name: str
+            age: int
+        ''')
+        self.flakes('''
+        class C:
+            name: str = 'Bob'
+            age: int = 18
+        ''')
+        self.flakes('''
+        def f():
+            name: str
+            age: int
+        ''')
+        self.flakes('''
+        def f():
+            name: str = 'Bob'
+            age: int = 18
+            foo: not_a_real_type = None
+        ''', m.UnusedVariable, m.UnusedVariable, m.UnusedVariable)
+        self.flakes('''
+        def f():
+            name: str
+            print(name)
+        ''', m.UndefinedName)
+        self.flakes('''
+        foo: not_a_real_type
+        ''', m.UndefinedName)
+        self.flakes('''
+        foo: not_a_real_type = None
+        ''', m.UndefinedName)
+        self.flakes('''
+        class C:
+            foo: not_a_real_type
+        ''', m.UndefinedName)
+        self.flakes('''
+        class C:
+            foo: not_a_real_type = None
+        ''', m.UndefinedName)
+        self.flakes('''
+        def f():
+            class C:
+                foo: not_a_real_type
+        ''', m.UndefinedName)
+        self.flakes('''
+        def f():
+            class C:
+                foo: not_a_real_type = None
+        ''', m.UndefinedName)