fix: make the switch to Int32 for population a TODO

src/covid-19/covid_19_utils/covid_19_utils/converters/__init__.py

@@ -83,8 +83,8 @@ class CaseConverterImpl:
     def _set_common_columns(self, df):
         """Use only the common columns; add missing ones when needed."""
         try:
-            # Int32 (as opposed to int32) is an intger type that allows nan
-            df["population"] = df.population.astype("Int32")
+            # TODO: Int32 (as opposed to int) is an intger type that allows nan
+            df["population"] = df.population.astype(int)
         except ValueError:
             pass