support pipx install

[jcrben]

By installing this with pipx install hello-tls I can avoid pulling the pyOpenSSL dependency into my global dependencies.

I had to expose a callable main() function from the __main__ module following the advice here https://stackoverflow.com/questions/27784271/how-can-i-use-setuptools-to-generate-a-console-scripts-entry-point-which-calls as trying to call the main module directly results in TypeError: 'module' object is not callable.

I tested this with test.pypi.org https://test.pypi.org/project/hello-tls-ben/:

python -m build
twine upload --repository-url https://test.pypi.org/legacy/ --username __token__ --password pypi-AgENdGVzdC5weXBpLm9yZwIkOWFjOWVlMTctYmUxZS00MmUzLTkyOWItOWNjZjBhYWEzNzc0AAIqWzMsImIxYjY3MWIzLWM0MDgtNDZhZi04OGJiLWYwNjlkNGEyMzBmMCJdAAAGIMx2j3U_it_jbE7jbAYvcZy3JyH_Xe_6Zof1qdvfwyd3 dist/*

pipx install --index-url https://test.pypi.org/simple --pip-args="--extra-index-url https://pypi.org/simple/" hello-tls-ben==0.3.17