BUG: pytables index expressions fail in Python 3.9
- [ y] I have checked that this issue has not already been reported. (Here - it [has in Debian](https://bugs.debian.org/972033))

- [ y] I have confirmed this bug exists on the latest version of pandas.

- [ y] (optional) I have confirmed this bug exists on the master branch of pandas.  (Though with a mix of Debian and pip dependencies, as pip doesn't have them all for 3.9 yet.)

---

#### Code Sample, a copy-pastable example

The tests in tests/io/pytables/test_store.py, or

```python
import pandas as pd;from pandas.io.pytables import HDFStore;s1=HDFStore("tmp1.h5","w");df=pd.DataFrame([[1,2,3],[4,5,6]],columns=['A','B','C']);s1.append("d1",df,data_columns=["B"]);df2=s1.select("d1","index>df.index[0]");print(type(df2.index[0]))
```

#### Problem description

In Python 3.9, HDFStore.Select fails if it involves an index expression, with this traceback:

```
self = <DatetimeArray>
['2000-01-03 00:00:00', '2000-01-04 00:00:00', '2000-01-05 00:00:00',
 '2000-01-06 00:00:00', '2000-01...2-08 00:00:00',
 '2000-02-09 00:00:00', '2000-02-10 00:00:00', '2000-02-11 00:00:00']
Length: 30, dtype: datetime64[ns]
key = 4

    def __getitem__(self, key):
        if lib.is_integer(key):
            # fast-path
            result = self._ndarray[key]
            if self.ndim == 1:
                return self._box_func(result)
            return self._from_backing_data(result)
    
        key = extract_array(key, extract_numpy=True)
        key = check_array_indexer(self, key)
>       result = self._ndarray[key]
E       IndexError: only integers, slices (`:`), ellipsis (`...`), numpy.newaxis (`None`) and integer or boolean arrays are valid indices

pandas/core/arrays/_mixins.py:200: IndexError
```
The debugger says key is a pandas.core.computation.pytables.Constant, while in Python 3.8 (where this works) it is a plain int.  The underlying cause may be [Python replacing ast.Index with bare values](https://docs.python.org/3/library/ast.html#ast.AST).

The CI may have missed this because it [skips optional dependencies on 3.9](https://github.com/pandas-dev/pandas/blob/master/.travis.yml#L97) (to avoid having to build them).

#### Possible fix
Warning: not fully tested.
```
--- a/pandas/core/computation/pytables.py
+++ b/pandas/core/computation/pytables.py

@@ -429,6 +429,10 @@ 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)
```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.9.0.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.19.0-11-amd64
Version          : #1 SMP Debian 4.19.146-1 (2020-09-17)
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : C
LANG             : C
LOCALE           : None.None

pandas           : 0+unknown
numpy            : 1.19.2
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 50.3.0
Cython           : 0.29.21
pytest           : 6.1.1
hypothesis       : 5.32.1
sphinx           : 3.2.1
blosc            : 1.9.2
feather          : None
xlsxwriter       : 1.1.2
lxml.etree       : 4.5.2
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.18.1
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : 1.2.1
fsspec           : 0.8.4
fastparquet      : None
gcsfs            : 0.7.1
matplotlib       : 3.3.2
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : 0.5.1
scipy            : 1.5.2
sqlalchemy       : 1.3.19
tables           : 3.6.1
tabulate         : 0.8.7
xarray           : None
xlrd             : 1.1.0
xlwt             : 1.3.0
numba            : None


</details>
