TST: adding failing tests for #3111

It is indeed the fast-path change that broke dtype behavior.
BUG: fixing issue #3111
Merge pull request #3118 from perimosocordiae/patch-2

BUG: sparse: fix dtype regression for [hv]stack

Fixes gh-3111
BUG: only normalize eigenvectors if they have been computed
Merge pull request #3122 from andrenarchy/eig-fix

BUG: linalg: fix division by zero in generalized eigenvalue problem
ENH: Add callback to slsqp in scipy.optimize
Merge pull request #3120 from matthias-k/slsqp-callback