UFL constraints not available for Firedrake

[tlroy]

UFL constraints are only available in fenics_adjoint/ufl_constraints. The example examples/stokes-topology/stokes-topology-rol-firedrake.py does not work as is. It's probably a simple fix to move ufl_constraints.py to pyadjoint instead.

I got it working for Firedrake by removing imports from fenics and firedrake_adjoint.types, which does not seem to exist.