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

Per the Python 3 docs, input() always returns a string:

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

> The function then reads a line from input, converts it to a
> string (stripping a trailing newline), and returns that.

The stubs from typeshed say the same:

https://github.com/python/typeshed/blob/5c69373890dfaf4f07f0638766fb0a4903352892/stdlib/3/builtins.pyi#L835

Here is the implementation from CPython with also shows a call to
PyUnicode_Decode on the result:

https://github.com/python/cpython/blob/3.7/Python/bltinmodule.c#L1960-L2143

As the value is always a string, there is nothing to decode. Therefore
the call to term_decode() unnecessary and can safely be removed.

With this in mind, must adjust quickstart tests to be more
representative.

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.