Release 1.6.0

[skupriienko]

Actions:

• Update CHANGELOG.md
• Update _version.py

Notes:

• A simple test:

python -m venv test_env1
source test_env1/bin/activate
pip install --index-url https://test.pypi.org/simple/ \
            --extra-index-url https://pypi.org/simple/ \
            mailgun==1.6.0rc1
python -c "from mailgun.client import AsyncClient"
python -c "from mailgun.client import Client"