Attribute error from `sklearn.base.is_regressor` due to missing `__sklearn_tags__` on nightly builds

### Describe the bug

Running the following script works without a warning in sklearn 1.7, but fails on the latest nightly release. It appears to be related to https://github.com/scikit-learn/scikit-learn/pull/31134, but I'm unsure why the issue only became apparent now.

### Steps/Code to Reproduce

```python
from sklearn.base import is_regressor

class MyEstimator:
    def __init__(self):
        ...

    def fit(self, X, y=None):
        self.is_fitted_ = True
        return self

    def predict(self, X):
        ...


est = MyEstimator()
is_regressor(est)
```

### Expected Results

I would expect the code to continue to run, possibly with a `DeprecationWarning`.

### Actual Results

```
Traceback (most recent call last):
  File "/Users/christian.bourjau/repos/scikit-learn-debug/.pixi/envs/nightlies/lib/python3.12/site-packages/sklearn/utils/_tags.py", line 275, in get_tags
    tags = estimator.__sklearn_tags__()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'MyEstimator' object has no attribute '__sklearn_tags__'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/christian.bourjau/repos/scikit-learn-debug/t.py", line 17, in <module>
    is_regressor(est)
  File "/Users/christian.bourjau/repos/scikit-learn-debug/.pixi/envs/nightlies/lib/python3.12/site-packages/sklearn/base.py", line 1238, in is_regressor
    return get_tags(estimator).estimator_type == "regressor"
           ^^^^^^^^^^^^^^^^^^^
  File "/Users/christian.bourjau/repos/scikit-learn-debug/.pixi/envs/nightlies/lib/python3.12/site-packages/sklearn/utils/_tags.py", line 283, in get_tags
    raise AttributeError(
AttributeError: The following error was raised: 'MyEstimator' object has no attribute '__sklearn_tags__'. It seems that there are no classes that implement `__sklearn_tags__` in the MRO and/or all classes in the MRO call `super().__sklearn_tags__()`. Make sure to inherit from `BaseEstimator` which implements `__sklearn_tags__` (or alternatively define `__sklearn_tags__` but we don't recommend this approach). Note that `BaseEstimator` needs to be on the right side of other Mixins in the inheritance order.
```

### Versions

```shell
System:
    python: 3.12.11 | packaged by conda-forge | (main, Jun  4 2025, 14:38:53) [Clang 18.1.8 ]
executable: /Users/christian.bourjau/repos/scikit-learn-debug/.pixi/envs/nightlies/bin/python
   machine: macOS-15.6.1-arm64-arm-64bit

Python dependencies:
      sklearn: 1.8.dev0
          pip: 25.2
   setuptools: 80.9.0
        numpy: 2.2.6
        scipy: 1.16.0
       Cython: 3.1.2
       pandas: 3.0.0.dev0+2502.g8476e0f756
   matplotlib: None
       joblib: 1.5.1
threadpoolctl: 3.6.0

Built with OpenMP: True

threadpoolctl info:
       user_api: blas
   internal_api: openblas
    num_threads: 16
         prefix: libopenblas
       filepath: /Users/christian.bourjau/repos/scikit-learn-debug/.pixi/envs/nightlies/lib/libopenblas.0.dylib
        version: 0.3.30
threading_layer: openmp
   architecture: VORTEX

       user_api: openmp
   internal_api: openmp
    num_threads: 16
         prefix: libomp
       filepath: /Users/christian.bourjau/repos/scikit-learn-debug/.pixi/envs/nightlies/lib/libomp.dylib
        version: None

       user_api: openmp
   internal_api: openmp
    num_threads: 16
         prefix: libomp
       filepath: /Users/christian.bourjau/repos/scikit-learn-debug/.pixi/envs/nightlies/lib/python3.12/site-packages/sklearn/.dylibs/libomp.dylib
        version: None
```

COMMENT:
> The reason we didn't raise a FutureWarning is that it would mean users get it w/o being able to do anything about it.

Yes, indeed, but they can at least report the problem to the library maintainers and silence the warning in the meantime while using the latest version of scikit-learn. I am worried that the next release will break a lot of user code and the only option the user will have will be to pin to an older version of scikit-learn while waiting for upstream to release a new, updated version of their library (which mike take time).

Labels: Bug