Inconsistency between sel and isel when working with slice

What happened:

Slice do not have the same effect when working with sel and isel.
The stop bound is not selected with isel while it is with sel.

What you expected to happen:
Either to select or not select the "stop" bound of the slice in both case.

Minimal Complete Verifiable Example:

import xarray as xr 
da = xr.Dataset()
da.coords["lat"] = [0, 1, 2, 3]
da["value"] = (('lat'),da["lat"].values+10)

print("Isel result is \n %s \n\n"%da.isel(lat=slice(1,2)))
print("Sel result is \n %s"%da.sel(lat=slice(1,2)))

Gives the following results

Isel result is 
 <xarray.Dataset>
Dimensions: (lat: 1)
Coordinates:
  * lat      (lat) int64 1
Data variables:
    value    (lat) int64 11 


Sel result is 
 <xarray.Dataset>
Dimensions:  (lat: 2)
Coordinates:
  * lat      (lat) int64 1 2
Data variables:
    value    (lat) int64 11 12

Anything else we need to know?:

Environment:

Start by running the supplied Python example and tracing the sel and isel entry points. Determine the intended stop-bound behavior for each indexing mode, then make the behavior consistent and add regression coverage for the reproducer.