Release version 1.0.0

After about a decade of Pyflakes, it's certainly no longer "pre 1.0"
software. There are no big changes since 0.9.2, really. It just felt
like the right thing to do.

New in this release:

  - Python 3.5 support. async/await statements in particular.

Sundry bugfixes:

  - test_api.py works on Windows now
  - Eliminated a false UnusedImport warning when the name has been
    declared "global"

Behind the scenes there have also been some adjustments to the CI
process to make sure 3.5 is tested, and a few more test cases added just
to better document why things are as they are.

I'd like to thank contributors to this release:

  - Eduard-Cristian Stefan
  - Ian Cordasco
  - Igor Davydenko
  - Ryne Everett
  - Steven Myint
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