Add note on mutable containers

[roberto-arista]

No description provided.

[roberto-arista]

Not sure why the tests did not work, I've only edited some documentation.
@benkiel @typesupply could you have a look?

[roberto-arista]

cc @typemytype

[typemytype]

two things:

• I guess we can remove python3.7 for test (not your problem)

• the other issue is an upload token you probably do not have set on your account: {'detail': ErrorDetail(string='Missing "owner" argument. Please upload with the Codecov repository upload token to resolve issue.', code='not_found')}

[benkiel]

Yes, it's an issue with fonttools dropping python 3.7. I'm going to merge and fix the tests...

[typemytype]

thanks Ben!!