Fix bug in adjacency_matrix(::SimpleGraph) for Julia 1.7

[mtfishman]

This fixes a bug in adjacency_matrix(::SimpleGraph) for Julia 1.7.

This PR is moved from sbromberger/LightGraphs.jl#1595 and fixes the issue described in sbromberger/LightGraphs.jl#1594.

As described in sbromberger/LightGraphs.jl#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.

Here is a minimal example:

julia> versioninfo()
Julia Version 1.7.0-rc1
Commit 9eade6195e (2021-09-12 06:45 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin19.6.0)
  CPU: Intel(R) Core(TM) m3-7Y32 CPU @ 1.10GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-12.0.1 (ORCJIT, skylake)

(@v1.7) pkg> st Graphs
      Status `~/.julia/environments/v1.7/Project.toml`
  [86223c79] Graphs v1.4.1 `~/.julia/dev/Graphs`

julia> using Graphs

julia> g = Graph(Edge.([1 => 2, 2 => 3]))
{3, 2} undirected simple Int64 graph

julia> adjacency_matrix(g)
3×3 SparseArrays.SparseMatrixCSC{Int64, Int64} with 4 stored entries:
 ⋅  1  ⋅
 1  ⋅  1
 ⋅  1  ⋅

julia> add_edge!(g, 1 => 1)
true

julia> adjacency_matrix(g)
3×3 SparseArrays.SparseMatrixCSC{Int64, Int64} with 5 stored entries:
 2  1  ⋅
 1  ⋅  1
 ⋅  1  ⋅

Without this fix I get:

(@v1.7) pkg> st Graphs
      Status `~/.julia/environments/v1.7/Project.toml`
  [86223c79] Graphs v1.4.1

julia> using Graphs

julia> g = Graph(Edge.([1 => 2, 2 => 3]))
{3, 2} undirected simple Int64 graph

julia> adjacency_matrix(g)
3×3 SparseArrays.SparseMatrixCSC{Int64, Int64} with 4 stored entries:
 ⋅  1  ⋅
 1  ⋅  1
 ⋅  1  ⋅

julia> add_edge!(g, 1 => 1)
true

julia> adjacency_matrix(g)
ERROR: ArgumentError: Illegal buffers for SparseMatrixCSC construction 3 [1, 3, 5, 6] [1, 2, 1, 3, 2] [1, 1, 1, 1, 1, 1]
Stacktrace:
 [1] SparseMatrixCSC
   @ /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.7/SparseArrays/src/sparsematrix.jl:29 [inlined]
 [2] SparseArrays.SparseMatrixCSC(m::Int64, n::Int64, colptr::Vector{Int64}, rowval::Vector{Int64}, nzval::Vector{Int64})
   @ SparseArrays /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.7/SparseArrays/src/sparsematrix.jl:44
 [3] _adjacency_matrix(g::SimpleGraph{Int64}, T::DataType, neighborfn::typeof(inneighbors), nzmult::Int64)
   @ Graphs.LinAlg ~/.julia/packages/Graphs/Mih78/src/linalg/spectral.jl:63
 [4] #adjacency_matrix#2
   @ ~/.julia/dev/Graphs/src/linalg/spectral.jl:25 [inlined]
 [5] adjacency_matrix (repeats 2 times)
   @ ~/.julia/dev/Graphs/src/linalg/spectral.jl:20 [inlined]
 [6] top-level scope
   @ REPL[24]:1

[mtfishman]

Also note that a variety of other tests fail on Julia 1.7, many related to sbromberger/LightGraphs.jl#1597.