Use Optional[T] instead of T | None for Python 3.9 compatibility

This converts the spelling to the latter form only for type
declarations consumed in beanquery, the others are left as an exercise
for someone else.

beancount/core/data.py

@@ -13,6 +13,7 @@
 from typing import Any
 from typing import Iterator
 from typing import NamedTuple
+from typing import Optional
 from typing import Protocol
 from typing import Union
 from typing import overload
@@ -111,7 +112,7 @@ class Open(NamedTuple):
     date: datetime.date
     account: Account
     currencies: list[Currency]
-    booking: Booking | None
+    booking: Optional[Booking]
 
 
 class Close(NamedTuple):
@@ -198,8 +199,8 @@ class Balance(NamedTuple):
     date: datetime.date
     account: Account
     amount: Amount
-    tolerance: Decimal | None
-    diff_amount: Amount | None
+    tolerance: Optional[Decimal]
+    diff_amount: Optional[Amount]
 
 
 class Posting(NamedTuple):
@@ -228,11 +229,11 @@ class Posting(NamedTuple):
     """
 
     account: Account
-    units: Amount | None
-    cost: Cost | CostSpec | None
-    price: Amount | None
-    flag: Flag | None
-    meta: Meta | None
+    units: Optional[Amount]
+    cost: Cost | Optional[CostSpec]
+    price: Optional[Amount]
+    flag: Optional[Flag]
+    meta: Optional[Meta]
 
 
 class Transaction(NamedTuple):
@@ -261,8 +262,8 @@ class Transaction(NamedTuple):
     meta: Meta
     date: datetime.date
     flag: Flag
-    payee: str | None
-    narration: str | None
+    payee: Optional[str]
+    narration: Optional[str]
     tags: frozenset[str]
     links: frozenset[str]
     postings: list[Posting]