fix: use Self to ensure correct return type for EmailPolicy cloning

[hoefling]

At the moment email.policy.EmailPolicy.clone() returns a Policy type, as can be reproduced with e.g.

$ mypy -c 'from email.policy import EmailPolicy; reveal_type(EmailPolicy().clone(utf8=True))'
<string>:1: note: Revealed type is "email.policy.Policy"
Success: no issues found in 1 source file

This differs at runtime:

$ python -c 'from email.policy import EmailPolicy; print(type(EmailPolicy().clone(utf8=True)))'
<class 'email.policy.EmailPolicy'>

This PR amends that.

[AlexWaygood]

Thanks!

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉