science/py-pymol: fix build on targets without OpenMP

armv7 still doesn't have it, so disable OpenMP there to make the port
build. The previous problem from PR 262208 seems to no longer occur.

Approved by: portmgr (build fix blanket)
MFH: 2023Q4

(cherry picked from commit c07e560b22f7edc7d26098be318af3c415311c9a)