premailer.premailer.ExternalFileLoadingError: Unable to load external
file 'assets/frappe/dist/css/email.bundle.HETZ7G4Y.css' because it's
explicitly not allowed
Unsupported new option introduced in premailer version 3.9.0
allow_loading_external_files=False
https://github.com/peterbe/premailer/blob/master/CHANGES.rst#390
Also, sort dependencies in python overlay by usage
