ecflow: Add version bound to pybind for ecflow >= 5.18

[eap]

Change: Constrain ecflow's pybind11 dependency to prevent inclusion of pybind11 >= 3

Rationale

When ecflow 5.18 was built against pybind11@3.0.2 I encountered the following error

$ python3 -c "import ecflow; print(ecflow.Client().version())"
5.18.0
munmap_chunk(): invalid pointer

Ecflow's cmake asks for 2.10.3 directly but in my tests I've found that pybind 2.x branch above 2.10 works. It is possible that prior versions also work but I haven't had the time to test that.

Anyhow, this updated spec appears to fix the issue by preventing the use of pybind11 3.x.