Add support for PEP 0526.

This makes it possible to assign variables like

    asdf: typing.List[int] = []

test/completion/pep0526_variables.py

@@ -0,0 +1,22 @@
+"""
+PEP 526 introduced a new way of using type annotations on variables. It was
+introduced in Python 3.6.
+"""
+# python >= 3.6
+
+import typing
+
+asdf = ''
+asdf: int
+# This is not necessarily correct, but for now this is ok (at least no error).
+#? int()
+asdf
+
+
+direct: int = NOT_DEFINED
+#? int()
+direct
+
+with_typing_module: typing.List[float] = NOT_DEFINED
+#? float()
+with_typing_module[0]