Question: when did the astros switch from the national league to the american league
[{'answer': '2013', 'provenance': [{'wikipedia_id': '13894', 'title': 'Houston Astros', 'start_paragraph_id': 1, 'start_character': 229, 'end_paragraph_id': 1, 'end_character': 233, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '13894', 'title': 'Houston Astros', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 371, 'bleu_score': 0.9684940092844061, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 1120991090544436912}, 'section': 'Section::::Abstract.'}]}]