Pyflakes

- Fix false RedefinedWhileUnused for submodule imports

- Warn against reusing exception names after the except: block on Python 3
- Improve the error messages for imports

- Allow main() to accept arguments.
- Support matrix-multiplication operator
- Validate ``__future__`` imports
- Fix doctest scope testing
- Warn for tuple assertions which are always true
- Warn for "import \*" not at module level on Python 3
- Catch many more kinds of SyntaxErrors
- Check PEP 498 f-strings
- (and a few more sundry bugfixes)

- Python 3.5 support. async/await statements in particular.
- Fix test_api.py on Windows.
- Eliminate a false UnusedImport warning when the name has been
declared "global"

- Fix a traceback when a global is defined in one scope, and used in another.

- Update NEWS.txt to include 0.9.0, which had been forgotten.