knuth-bendix: fix helper to_latin1 again

Author: james-d-mitchell

src/knuth-bendix.cpp

@@ -54,10 +54,14 @@ namespace libsemigroups {
   namespace {
     std::string to_latin1(std::string const& u) {
       static py::object bytes;
-      try {
-        bytes = py::getattr(py::globals()["__builtins__"], "bytes");
-      } catch (py::error_already_set& e) {
-        bytes = py::globals()["__builtins__"]["bytes"];
+      static bool       first_call = true;
+      if (first_call) {
+        first_call = false;
+        try {
+          bytes = py::getattr(py::globals()["__builtins__"], "bytes");
+        } catch (py::error_already_set& e) {
+          bytes = py::globals()["__builtins__"]["bytes"];
+        }
       }
       return PyBytes_AS_STRING(bytes(py::str(u), "latin1").ptr());
     }