Use python 3.9 for running tests and building docs
Swtich FreeBoundary and Case classes from attrs classes to regular classes. For some reason sphinx-build would complain about not being able to infer the types of arguments in methods belonging to these attrs classes
Changes made:
FreeBoundary
andCase
classes from attrs classes to regular classes. For some reason sphinx-build would complain about not being able to infer the types of arguments in methods belonging to these attrs classes