We can't take the complement of an immutable graph:

sage: Gamma = graphs.PathGraph(5).copy(immutable=True)
sage: Gamma.complement()
NotImplementedError

which is because copy(Gamma) doesn't return a mutable copy.

Depends on #15681

CC: @nathanncohen

Component: graph theory

Author: Travis Scrimshaw

Branch/Commit: public/graphs/complement-15669 @ c393b58

Reviewer: Nathann Cohen

Issue created by migration from https://trac.sagemath.org/ticket/15669