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.

tests/test_build_gettext.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 """
     test_build_gettext
     ~~~~~~~~~~~~~~~~~~