Fix pyflakes error with msg/message in assert_dicts_equal

PASSED.  236 tests / 100 cases: 236 passed, 0 failed.  (Total test time 4.56s)

```
[15:06:34]rlipman@dev13:(fix-pyflakes-assert):~/dev/Testify$ find testify/ -name __init__.py -o -name "*.py" -print | xargs pyflakes
(no output)
```