Github Pages allows you to link a custom domain to your Github Pages
site by adding a `CNAME` file at the top-level of your `docs` directory.
The `githubpages` extension already inserts a `.nojekyll` file in the
`docs` directory, so it's a good place to add support for this `CNAME`
file as well.

In Python 3, the default encoding of source files is utf-8. The encoding
cookie is now unnecessary and redundant so remove it. For more details,
see the docs:

https://docs.python.org/3/howto/unicode.html#the-string-type

> The default encoding for Python source code is UTF-8, so you can
> simply include a Unicode character in a string literal ...

Includes a fix for the flake8 header checks to stop expecting an
encoding cookie.