Integrate setuptools_scm

Integrated automatic version information generation using setuptools_scm in pyproject.toml file.

To check that things work as expected, execute the following commands, while at the root of the current git repository:

# Create and activate virtualenv
python3 -m venv venv
. venv/bin/activate

# Upgrade pip
pip install -U pip

# Install my-awesome-app (the `-e` option is to do a developer install, not strictly necessary but handy):
pip install -e .

# Check it works
python -c 'import my_awesome_app; print(my_awesome_app.__version__)'