Frobenius endomorphisms of p-adic fields pickle incorrectly

[pbruin]

In SageMath 9.3.beta1:

sage: R.<x> = QQ[]
sage: U.<a> = Qp(2).extension(x^2 + x + 1)
sage: F = U.frobenius_endomorphism(); F
Frobenius endomorphism on 2-adic Unramified Extension Field in a defined by x^2 + x + 1 lifting a |--> a^2 on the residue field
sage: copy(F)
Identity endomorphism of 2-adic Unramified Extension Field in a defined by x^2 + x + 1
sage: loads(dumps(F))
Identity endomorphism of 2-adic Unramified Extension Field in a defined by x^2 + x + 1

This is fixed by implementing suitable _extra_slots() and _update_slots() methods.

[tscrim]

LGTM.