prettytableについて質問です。
以下の場合、City nameは左揃え、AreaとAnnual Rainfallは右揃えになるんですが、
ヘッダのみ中央揃えにするにはどう書けばよいんでしょうか？

$ vim alignment.py
#!/usr/bin/python3

from prettytable import PrettyTable

x = PrettyTable()

x.field_names = ["City name", "Area", "Population", "Annual Rainfall"]

x.align["City name"] = "l"
x.align["Area"] = "r"
x.align["Annual Rainfall"] = "r"

x.add_row(["Adelaide", 1295, 1158259, 600.5])
x.add_row(["Brisbane", 5905, 1857594, 1146.4])
x.add_row(["Darwin", 112, 120900, 1714.7])

print(x)

$ ./alignment.py
+-----------+------+------------+-----------------+
| City name | Area | Population | Annual Rainfall |
+-----------+------+------------+-----------------+
| Adelaide | 1295 | 1158259 | 600.5 |
| Brisbane | 5905 | 1857594 | 1146.4 |
| Darwin | 112 | 120900 | 1714.7 |
+-----------+------+------------+-----------------+