PERF: use _get_axis_number instead of if axis in [...]: #43048
 Added _get_axis_number in test_at_time.py as per the standards

``` 
 if axis in ["index", 0]:              #<- master
      expected = ts.loc[indices, :]
 elif axis in ["columns", 1]:
      expected = ts.loc[:, indices]

 axis = self.data._get_axis_number(axis)   #<- PR
 expected = ts.loc[indices, :] if axis == 0 else ts.loc[:, indices]
```