Python 3 defaults to UTF-8 source encoding

This replaces an out-dated guideline about declaring the source encoding
at the top of each source file.

doc/unicode.rst

@@ -16,14 +16,9 @@ To that end, adhere to the following guidelines:
 
 + Use Unicode strings and Unicode literals everywhere.
 
-+ If you have non-ASCII characters in a Python source file (e.g., in Unicode
-  literals such as ``'α'``), you need to
-  `declare the file encoding <https://www.python.org/dev/peps/pep-0263/>`__ at
-  the top of the file:
++ Ensure that Python source files are saved as UTF-8 (the `default encoding <https://peps.python.org/pep-3120/>`__).
 
-  .. code-block:: python
-
-     # -*- coding: utf-8 -*-
+  + If you need to use a different encoding for Python source files, you must `declare this encoding <https://www.python.org/dev/peps/pep-0263/>`__ at the top of each file.
 
 + Encode Unicode text into UTF-8 when writing to disk: