F812 fails under Python 3

Under both Python 2 and 3, I expect the F812 warning to be reported.

I originally found this via TravisCI testing, see https://travis-ci.org/peterjc/biopython/builds/225261622 for example, before preparing this minimal test case. Originally logged against flake8 as https://gitlab.com/pycqa/flake8/issues/326

I am using pyflakes 1.5.0 installed with pip under macOS using either Apple provided Python 2.7, or a self-compiled Python 3.4.

Test script and its output:

```
$ cat test_F812.py
x = 3

y = [chr(x + 32) for x in range(3)]

print("The value of x should be 2 or 3 (matching your version of Python):")
print(x)
$ python test_F812.py
The value of x should be 2 or 3 (matching your version of Python):
2
$ python3 test_F812.py
The value of x should be 2 or 3 (matching your version of Python):
3
```

Expected output as seen on Python 2.7,

```
$ /usr/local/bin/pyflakes --version
1.5.0
$ /usr/local/bin/pyflakes test_F812.py ; echo "Return code $?"
test_F812.py:3: list comprehension redefines 'x' from line 1
Return code 1
```

or to show explicitly this is under Python 2,

```
$ python --version
Python 2.7.10
$ python -m pyflakes --version
1.5.0
$ python -m pyflakes test_F812.py ; echo "Return code $?"
test_F812.py:3: list comprehension redefines 'x' from line 1
Return code 1
```

Problem output as seen on Python 3.4,

```
$ pyflakes --version
1.5.0
$ pyflakes test_F812.py ; echo "Return code $?"
Return code 0
```

or to show explicitly this is under Python 3,

```
$ python3 --version
Python 3.4.1
$ python3 -m pyflakes --version
1.5.0
$ python3 -m pyflakes test_F812.py ; echo "Return code $?"
Return code 0
```