pybind11 back to 2.5.0 to avoid sphinx build problem

docs/source/_templates/layout.html

-{% extends "!layout.html" %}
-
-{% block menu %}
-  {{ super() }}
-  <a href="/spam/py-modindex.html">Module Index</a>
-{% endblock %}

requirements.txt

@@ -22,7 +22,8 @@ progressbar==2.5
 gmsh==4.8.0 # not needed if you can install it with apt install python3-gmsh
 meshio==4.3.11 # Python >=3.6
 
-pybind11==2.6.2 # Python !=3.0, !=3.1, !=3.2, !=3.3, !=3.4, >=2.7
+# Anything later than this causes an abort at the end of sphinx build
+pybind11==2.5.0 # Python !=3.0, !=3.1, !=3.2, !=3.3, !=3.4, >=2.7
 
 # multigrid solver for random walker segmentation
 pyamg==4.0.0

[Emmanuel Roubin]

good catch