Add support for Python 3.8

[hugovk]

Here's a quick checklist that should be present in PRs:

• Make sure to include reasonable tests for your change if necessary

• We use towncrier for changelog management, so please add a news file into the changelog folder following these guidelines:

  • Name it $issue_id.$type for example 588.bugfix

  • If you don't have an issue_id change it to the PR id after creating it

  • Ensure type is one of removal, feature, bugfix, vendor, doc or trivial

  • Make sure to use full sentences with correct case and punctuation, for example:

    Fix issue with non-ascii contents in doctest text files.
    

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Also rename appveyor.yml to .appveyor.yml. Let's group it with the other dotfiles for consistency and less clutter.

AppVeyor supports dot-file-style YAML named .appveyor.yml as is.

https://www.appveyor.com/docs/build-configuration/#yaml-file-alternative-naming

[nicoddemus]

Thanks @hugovk!