chore(manifest): remove unneeded directives

[stegayet]

Cleaning before #1705.

Description

This PR removes unneeded directives in MANIFEST.in file, cf. https://packaging.python.org/en/latest/guides/using-manifest-in/.

Exclude directives have also been removed and instead added to .gitignore as they are compiled Python files or docs build artifacts.

How this has been tested ?

Used the following script to check sdist content diff between both versions:

git checkout main
python setup.py sdist
tar -tvf dist/kombu-5.3.0b3.tar.gz > old_archive_content.txt
git checkout chore/clean-manifest-file
python setup.py sdist
tar -tvf dist/kombu-5.3.0b3.tar.gz > new_archive_content.txt
diff new_archive_content.txt old_archive_content.txt > diff.txt

The last command returns nothing.

[auvipy]

I need to check them properly before merge/remove them