This fixes #271.

Forgot to push the commit that increments to 1.5.0 when I made the
release, and some changes have been made since then :(

* Don't test against file perms when running as root

test_permissionDenied tests file modes by creating a temporary file, then sets the mode to 0000 and attempts to process it and see if and permission denied error is generated.  This never happens when the unit tests are run as root since it can open files regardless of file permissions.

* Fix up UID skip in tests for Windows

Even though variable annotations in function scope aren't evaluated at
runtime it's still useful for static analysis tools to process them and
catch some issues (and not report some things that aren't issues).

Let's take the following code:

     from typing import Any

     def fun():
          a: Any

Previously pyflakes would report Any to be unused:

     test.py:1: 'typing.Any' imported but unused

With this patch it's no longer the case.

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.

This was inconsistent with the other error messages. This is useful as
it allows easy detecting of the offending item for editor plugins by
simply searching in the message for quotes.

format strings (PEP 498) defines a new AST node, JOINEDSTR which
contains a list of string expressions.