MAINT: use COO setdiag to help clarify under-tested csgraph.laplacian
MAINT: restore asymmetric laplacian behavior and change docs accordingly
TST: add test coverage of laplacian ValueError
PEP8
PEP8: change indentation
PEP8: now does the closing bracket match visual indentation?
Merge pull request #3827 from argriffing/coo-diag

BUG: sparse.csgraph: laplacian fixups