BUG #19860 Corrected use of mixed indexes with .at
`.at` incorrectly disallowed the use of integer indexes when a mixed index was used
```
s = Series([1,2,3,4,5],index=['a','b','c',1,2])
s.at['a'] # returns 1
s.at[1]   # returns 4
s.at[4]   # raises KeyError, doesn't do fallback indexing
```

Made sure fallback indexing doesn't happen on mixed indexes

- [X] closes #19860
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry
