Cleanup: remove "<pep8 compliant>" from headers

It can be assumed that all scripts comply with basic pep8 formatting
regarding white-space, indentation etc.

Also remove note in best practices page & update `tests/python/pep8.py`.

If we want to exclude some scripts from make format,
this can be done by adding them to `ignore_files` in:
source/tools/utils_maintenance/autopep8_format_paths.py

Or using `# nopep8` for to ignore for individual lines.

Ref T98554

release/scripts/modules/keyingsets_utils.py

@@ -1,7 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 
-# <pep8-80 compliant>
-
 # This file defines a set of methods that are useful for various
 # Relative Keying Set (RKS) related operations, such as: callbacks
 # for polling, iterator callbacks, and also generate callbacks.