Remove unnecessary encoding cookie from Python source files

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.

With only a single text type across supported Python versions, the
string_classes is no longer necessary.

Internally, all uses were converted to the value `[str]`. For
.add_config_value() uses that also supply a default string, the type is
inferred.