Bug#972015: Bug#972033: python3.9 pandas

[Rebecca N. Palmer]

Control: tags -1 patch

The underlying cause (and reason this is 3.9-specific) appears to be the 
mostly-removal of ast.Index and its replacement by a bare value.

This appears to fix it, though I'm not yet sure if it's a good idea:

--- a/pandas/core/computation/pytables.py
+++ b/pandas/core/computation/pytables.py
@@ -425,6 +425,10 @@ class PyTablesExprVisitor(BaseExprVisito
              value = value.value
          except AttributeError:
              pass
+        try:
+            slobj = slobj.value
+        except AttributeError:
+            pass

          try:
              return self.const_type(value[slobj], self.env)