Release 1.8.0

[skupriienko-mailgun]

Actions:

• Update CHANGELOG.md
• Update _version.py
• Build the package from the source
• Fix minor issues and typos
  • Fix and improve markdown format in README.md
  • Fix AsyncClient._client regression bug

Notes:

• A simple test:

python3 -m venv test_mailgun180rc2

source test_mailgun180rc2/bin/activate

pip3 install --upgrade pip
pip3 install mailgun==1.8.0rc2

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 v1.8.0rc2