Data*.groupby with sequences yields scalars for len-1 sequences

What happened?

When executing the following code:

import xarray as xr
import numpy as np

mda = xr.DataArray(np.random.rand(6, 3), [("x", list("abcabc")), ("y", range(3))])
print(mda)

print("scalar groupby")
for value, group in mda.groupby("x"):
    print(value)
    assert isinstance(value, str)
    break

print("multiple groupby")
for value, group in mda.groupby(["x", "y"]):
    print(value)
    assert isinstance(value, tuple)
    break

print("multiple *scalar* groupby")
for value, group in mda.groupby(["x"]):
    print(value)
    assert isinstance(value, tuple)
    break

I would have expected the last invocation to yield a value of a tuple of length 1. When doing variable groupby segments with sequences of varying length it seems important to keep the dimensions of the value returned.

What did you expect to happen?

I would have expected the last value (in multiple scalar groupby) to yield a (value,) instead.

Minimal Complete Verifiable Example

import xarray as xr
import numpy as np

mda = xr.DataArray(np.random.rand(6, 3), [("x", list("abcabc")), ("y", range(3))])
print(mda)

for value, group in mda.groupby(["x"]):
    assert isinstance(value, tuple)

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

Anything else we need to know?

No response

Environment

Start by reproducing the behavior at the DataArray.groupby(["x"]) entry point using the minimal example. Trace how grouped keys are produced for a one-element sequence and verify that the completed behavior preserves a one-item tuple, with regression coverage for the reported case.