Add Python 3 support for validate-modules test. (#21195)

* Improve PEP8 compatibility

* Fix Python 3 incompatibility

Is prohibited to mutate OrderedDict during iteration through it so
is better to add records with error or warning to empty dictionary
instead of delete records from copy of dictionary during iterating.

* Decode output of subprocess from bytes to unicode.

* Add Python 3 support for validate-modules test.

Fix #18367

test/sanity/code-smell/shebang.sh

@@ -7,7 +7,6 @@ grep '^#!' -rIn . \
     -e '^\./lib/ansible/modules/' \
     -e '^\./test/integration/targets/[^/]*/library/[^/]*:#!powershell$' \
     -e '^\./test/integration/targets/[^/]*/library/[^/]*:#!/usr/bin/python$' \
-    -e '^\./test/sanity/validate-modules/validate-modules:#!/usr/bin/env python2$' \
     -e '^\./hacking/cherrypick.py:#!/usr/bin/env python3$' \
     -e ':#!/bin/sh$' \
     -e ':#!/bin/bash( -[eux]|$)' \