DEP: add actual DeprecationWarning for sym_pos-keyword of scipy.linalg.solve

[j-bowhay]

Reference issue

Closes #15735

What does this implement/fix?

The sym_pos keyword of scipy.linalg.solve has been deprecated in the documentation but no explicit warning is raised. This pr raises a deprecation warning so that sym_pos can be removed in SciPy 1.11.0.

Additional information

[ilayn]

Thanks @j-bowhay. There are a few instances of sym_pos in the SciPy codebase too; two that I could detect are

scipy/scipy/optimize/_linprog_ip.py

Line 117 in 8a64c93

return sp.linalg.solve(M, r, sym_pos=sym_pos)

scipy/scipy/signal/_fir_filter_design.py

Line 1053 in 071c570

a = solve(Q, b, sym_pos=True, check_finite=False)

which needs to be converted to assume_a = 'pos'.

[ilayn]

Looks good now. Thanks @j-bowhay , @h-vetinari, @mdhaber .