sklearn deprecated

Machine learning engineer lab students are instructed to install `sklearn` via pip.

https://github.com/GoogleCloudPlatform/training-data-analyst/blob/1e4981391bb531fda25d7b6158fc0a759926e036/courses/machine_learning/deepdive2/feature_engineering/labs/3_keras_basic_feat_eng-lab.ipynb#L73

Today's results are:

```
Collecting sklearn
Downloading sklearn-0.0.post9.tar.gz (3.6 kB)
Preparing metadata (setup.py) ... error
error: subprocess-exited-with-error

× python setup.py egg_info did not run successfully.
│ exit code: 1
╰─> [18 lines of output]
The 'sklearn' PyPI package is deprecated, use 'scikit-learn'
rather than 'sklearn' for pip commands.

Here is how to fix this error in the main use cases:
- use 'pip install scikit-learn' rather than 'pip install sklearn'
- replace 'sklearn' by 'scikit-learn' in your pip requirements files
(requirements.txt, setup.py, setup.cfg, Pipfile, etc ...)
- if the 'sklearn' package is used by one of your dependencies,
it would be great if you take some time to track which package uses
'sklearn' instead of 'scikit-learn' and report it to their issue tracker
- as a last resort, set the environment variable
SKLEARN_ALLOW_DEPRECATED_SKLEARN_PACKAGE_INSTALL=True to avoid this error

More information is available at
https://github.com/scikit-learn/sklearn-pypi-package

If the previous advice does not cover your use case, feel free to report it at
https://github.com/scikit-learn/sklearn-pypi-package/issues/new
[end of output]
```

`scikit-learn` appears to be known already to the maintainers as the successor library.