I think you’ve almost got it. Is this what you want?
from mpl_toolkits.mplot3d import Axes3D
from mpl_toolkits.mplot3d.art3d import Poly3DCollection
import matplotlib.pyplot as plt
fig = plt.figure()
ax = Axes3D(fig, auto_add_to_figure=False)
fig.add_axes(ax)
x = [0,1,1,0]
y = [0,0,1,1]
z = [0,1,0,1]
verts = [list(zip(x,y,z))]
ax.add_collection3d(Poly3DCollection(verts))
plt.show()
You might also be interested in art3d.pathpatch_2d_to_3d.