The source strength is specified with the strength= argument to Source(). Since it defaults to 1, if you want a unit strength source you don’t need to specify it. To do a unit circle source in the x-y plane at z=0, the following should work:
r = openmc.stats.PowerLaw(0.0, radius, 1.0)
phi = openmc.stats.Uniform(0.0, 2*pi)
z = openmc.stats.Discrete([1.0], [0.0])
spatial_dist = openmc.stats.CylindricalIndependent(r, phi, z)
source = openmc.Source(space=spatial_dist, ...)
Right now, it’s only possible to do this in the x-y plane unless you wanted to write your own custom source routine.