This removes the last use of the six package allowing Sphinx to remove
it as a dependency.

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.

All code passes type str to the method. Per the type signature, only str
is allowed.

The open() function opens files in read-only text mode by default. Drop
the mode argument to be slightly simpler and more idiomatic.

https://docs.python.org/3/library/functions.html#open

> The default mode is 'r' (open for reading text, synonym of 'rt').