invoke and tasks.py

[edublancas]

Invoke uses a tasks.py file by default, but this can be confusing. It's possible to change the name but then we have to pass the -c arg.

It's possible to override the default, but this requires adding a config file specific to invoke. There's an issue pyinvoke/invoke#537 to support pyproject.toml, but there hasn't been any progress recently.