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

tests/python/rst_to_doctree_mini.py

@@ -1,7 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 
-# <pep8 compliant>
-
 # Module with function to extract a doctree from an reStructuredText file.
 # Named 'Mini' because we only parse the minimum data needed to check
 # Python classes, methods and attributes match up to those in existing modules.