Annual conversion in .grain() not working for pandas<2.0.0 [BUG]

[cdietrich215]

Describe the bug Getting an error when converting to an annual grain when pandas <2.0.0 using the .grain() method. I believe this was caused by the release of chainladder version 0.8.21

To Reproduce

The following code causes an error when chainladder verion = 0.8.23 and pandas version = 1.5.2

import chainladder as cl
import pandas as pd

tri = cl.load_sample('prism')['Incurred'].sum()
tri.grain('OYDY')