books: Introduce Year type alias.

This is going to be repeated a lot as we write book-loading methods.

conservancy_beancount/books.py

@@ -23,6 +23,8 @@ from typing import (
     Union,
 )
 
+Year = Union[int, datetime.date]
+
 class FiscalYear(NamedTuple):
     month: int = 3
     day: int = 1
@@ -35,10 +37,7 @@ class FiscalYear(NamedTuple):
         else:
             return date.year
 
-    def range(self,
-              from_fy: Union[int, datetime.date],
-              to_fy: Union[int, datetime.date, None]=None,
-    ) -> Iterable[int]:
+    def range(self, from_fy: Year, to_fy: Optional[Year]=None) -> Iterable[int]:
         """Return a range of fiscal years
 
         Both arguments can be either a year (represented as an integer) or a