Release 1.9.1

[skupriienko-mailgun]

Actions:

• Update CHANGELOG.md
• Update _version.py
• Build the package from the source
• Test mailgun.examples
• Update hooks in .pre-commit-config.yaml
• Fix minor issues and bugs

Notes:

• A simple test:

python3 -m venv test_mailgun191rc1

source test_mailgun191rc1/bin/activate

pip3 install --upgrade pip
pip3 install mailgun==1.9.1rc1

python3 -c "from mailgun.client import Client"
python3 -c "from mailgun.client import AsyncClient"
python3 -c "import mailgun; from importlib.metadata import version; print(f'Successfully installed v{version(\"mailgun\")}')"

# output: 
# Successfully installed v1.9.1rc1