By Brendan O’Brien (Reuters) – The celebrated groundhog known as Punxsutawney Phil emerged from his Pennsylvania burrow on Thursday and saw his shadow, a sign of six more weeks of frosty weather in North America, which so far has enjoyed one of the mildest winters on record. Thousands of revelers gathered at dawn in Punxsutawney, a small town northeast of Pittsburgh, to celebrate Groundhog Day and watch Phil climb out of his tree stump to offer his prognostication of what lies ahead in a quirky yearly ritual. “I see a shadow on my stage, and so no matter how you measure, it’s six more weeks of…