Almost - need to be strings!

packages/py-spinnfrontendcommon/package.py

@@ -37,7 +37,7 @@ class PySpinnfrontendcommon(PythonPackage):
         # Work out the python version installed, so we know where to put
         # the java code!
         python_spec = find("python@3.7:")[0]
-        python_version = ".".join(python_spec.version.version[0:2])
+        python_version = ".".join(str(v) for v in python_spec.version.version[0:2])
         python_lib = f"python{python_version}"
         site_packages = prefix.lib.join(python_lib).site_packages
         install_tree("JavaSpiNNaker", site_packages)