[bugfix] make environment variable a str

python/dune/perftool/__init__.py

 # Disallow numpy to use multithreading - it breaks parallel code generation
 # on machines with large core count.
 import os
-os.environ["OMP_NUM_THREADS"] = 1
+os.environ["OMP_NUM_THREADS"] = "1"
 
 # Trigger imports that involve monkey patching!
 import dune.perftool.loopy.symbolic