feat: add Series.case_when()

[tswast]

feat: add DataFrame.__delitem__
docs: add logistic regression samples

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Fixes #<issue_number_goes_here> 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 5 region tags.

• samples/snippets/logistic_regression_prediction_test.py:23, tag bigquery_dataframes_logistic_regression_prediction_examine
• samples/snippets/logistic_regression_prediction_test.py:50, tag bigquery_dataframes_logistic_regression_prediction_prepare
• samples/snippets/logistic_regression_prediction_test.py:75, tag bigquery_dataframes_logistic_regression_prediction_create_model
• samples/snippets/logistic_regression_prediction_test.py:92, tag bigquery_dataframes_logistic_regression_prediction_evaluate_model
• samples/snippets/logistic_regression_prediction_test.py:114, tag bigquery_dataframes_logistic_regression_prediction_predict_income_bracket

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[tswast]

Mailed cl 632273853 for samples failure, though I'm not sure why only Python 3.9 failed.