Polars: py-0.20.21 Release

🚀 Performance improvements

• Fix regression that led to using only a single thread (#15667)

✨ Enhancements

• add ewm_mean_by (#15638)

🐞 Bug fixes

• Ensure profile of simple-projection only take own runtime (#15671)
• Panic if invalid array in object (#15664)
• Ensure 'CachedSchema' doesn't get synced between plans (#15661)
• group_by multiple null columns produce phantom row (#15659)
• rolling_* aggs were behaving as if they return scalars in group-by (#15657)
• Correct the unsoundness slice range of arr.min/max (#15654)
• list.mean fast path shouldn't produce NaN (#15652)

📖 Documentation

• Add missing deprecation warning to DataFrame.replace (#15612)
• Fix typo in legacy install instructions (#15662)

🛠️ Other improvements

• make Duration.is_constant_duration less strict for non-timezone-aware case (#15639)
• Fix some typos in comments (#15665)
• remove unnecessary unsafe in list mean/sum (#15660)
• fixup failing test due to offset deprecation in upsample (#15636)

Thank you to all our contributors for making this release possible! @MarcoGorelli, @Priyansh4444, @StevenMia, @eitsupi, @itamarst, @mcrumiller, @orlp, @reswqa, @ritchie46 and @stinodego