FWIW, now the PDBTools.jl package is updated (to version 3.1.0) and the selection syntax is more powerful, accepting the original code you posted here, and also parentheses, etc. You could do now:
cm.select(atoms, "(residue 1 3 7 5) and backbone")
#or
cm.select(atoms, "protein and resnum 2 7 39 81")
for example.
Just update the packages using
import juliacall as jl
jl.Pkg.update()