Pyflakes

- Fix PyPy2 Windows IntegrationTests
- Check for duplicate dictionary keys
- Fix TestMain tests on Windows
- Fix "continue" and "break" checks ignoring py3.5's "async for" loop

- Fix TypeError when processing relative imports

- Avoid traceback when exception is del-ed in except

- 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)