Release 1.6.0

[skupriienko]

Actions:

• Update CHANGELOG.md
• Update _version.py
• Build the package from the source
• Fix minor issues and typos

Notes:

• A simple test:

python -m venv test_mailjet160

source test_mailjet160/bin/activate

pip install mailjet-rest==1.6.0rc1

python -c "from mailjet_rest.client import Client"
python -c "import mailjet_rest; from importlib.metadata import version; print(f'Successfully installed v{version(\"mailjet_rest\")}')"