To control trimming doctest flags manually, this adds new options
:trim-doctest-flags: and :no-trim-doctest-flags: to doctest
directives. It helps to describes doctest module itself in python
doc (see #6698).
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.
