Fix bug when making an `adjacency_matrix` from a `SimpleGraph` with self-loops in Julia 1.7

[mtfishman]

Fix bug when making an adjacency_matrix from a SimpleGraph with self-loops in Julia 1.7

As described in #1594, Julia 1.7 adds a stricter check on the SparseMatrixCSC constructor for consistency about the number of nonzero elements input. The number of nonzero elements are currently being overcounted in the case of SimpleGraph with self-loops, this should fix that overcounting.

Fixes #1594.

[mtfishman]

Note that when I run the tests locally using Julia 1.7, there are other tests that fail, so it looks like the package needs some other updates for Julia 1.7.

[Tokazama]

Would be great if these could be fixed soon. I'm trying to ensure improvements are stable across Julia versions for #1596 and these errors make it pretty difficult.