More 3.9 fixes

Author: benjeffery

python/tskit/jit/numba.py

@@ -17,7 +17,11 @@ def jitdataclass(cls):
     del dc_cls.__dataclass_params__
     del dc_cls.__dataclass_fields__
     del dc_cls.__repr__
-    del dc_cls.__replace__
+    try:
+        del dc_cls.__replace__
+    except AttributeError:
+        # __replace__ is not available in Python < 3.10
+        pass
     del dc_cls.__match_args__
     return numba.experimental.jitclass(dc_cls)