Fix pytables expressions with Python 3.9

ast.Index has been replaced by a bare value, so we need to do the
conversion from Constant to int

Author: Rebecca N. Palmer <rebecca_palmer@zoho.com>
Bug-Debian: https://bugs.debian.org/972015
Forwarded: pandas-dev/pandas#37217


Gbp-Pq: Name python39_compat.patch

pandas/core/computation/pytables.py

@@ -425,6 +425,10 @@ def visit_Subscript(self, node, **kwargs):
             value = value.value
         except AttributeError:
             pass
+        try:
+            slobj = slobj.value
+        except AttributeError:
+            pass
 
         try:
             return self.const_type(value[slobj], self.env)