Use updated pandas frequency string

Author: jessegrabowski

pymc_experimental/tests/statespace/utilities/test_helpers.py

@@ -20,7 +20,7 @@
 
 def load_nile_test_data():
     nile = pd.read_csv("pymc_experimental/tests/statespace/test_data/nile.csv", dtype={"x": floatX})
-    nile.index = pd.date_range(start="1871-01-01", end="1970-01-01", freq="AS-Jan")
+    nile.index = pd.date_range(start="1871-01-01", end="1970-01-01", freq="YS-JAN")
     nile.rename(columns={"x": "height"}, inplace=True)
     nile = (nile - nile.mean()) / nile.std()
     nile = nile.astype(floatX)