fix: Change joblib import to top-level
scikit-learn 0.23 removed `sklearn.external.joblib` in favour of the top-level `joblib` package. This causes this example for the AI platform to fail if run on the latest runtime.
# [START setup]
import datetime
import pandas as pd
+import joblib
from google.cloud import storage
from sklearn.ensemble import RandomForestClassifier
-from sklearn.externals import joblib
from sklearn.feature_selection import SelectKBest
from sklearn.pipeline import FeatureUnion
from sklearn.pipeline import Pipeline
