Minor fix: replace deprecated code

[julianharty]

Context

Let's keep the code relatively clean and current while we're also exploring and experimenting.

What to address

The auto-generated code generated a couple of FutureWarnings when run in python 3.10, as follows:

python tsv-parsing-experiment.py                                    
.../tsv-parsing-experiment.py:33: FutureWarning: Series.fillna with 'method' is deprecated and will raise in a future version. Use obj.ffill() or obj.bfill() instead.
  df['a'] = df['a'].fillna(method='ffill')
.../tsv-parsing-experiment.py:34: FutureWarning: Series.fillna with 'method' is deprecated and will raise in a future version. Use obj.ffill() or obj.bfill() instead.
  df['b'] = df['b'].fillna(method='ffill')