TST: suppress 1485 warnings issued by xml parser
So we don't have to scroll through tons of 

```
pandas/tests/io/excel/test_xlrd.py::test_excel_table_sheet_by_index[.xlsm]
  /usr/local/lib/python3.7/site-packages/xlrd/xlsx.py:312: PendingDeprecationWarning: This method will be removed in future versions.  Use 'tree.iter()' or 'list(tree.iter())' instead.
    for elem in self.tree.iter() if Element_has_iter else self.tree.getiterator():
```
related to #27016 Thanks! Do we know when that will be enforced? 3.9? So xlrd will be totally broken by then? I'm not sure. It's the `getiterator` function in `xml.etree.ElementTree` which was deprecated back in `3.2`. It's at least still available in 3.8

Also I can't guarantee it will break xlrd when removed as xlrd relies on a few different underlying parsers and may fall back in case of failure. In any case it's not something that will be changed since that package isn't maintained any more