Make BaseTzInfo inherited methods pos-only (#13650)

These methods are already positional-only in `datetime.tzinfo`. Leaving
them pos-or-kw fail stubtest when running with Python 3.13.

Author: srittau

stubs/pytz/pytz/tzinfo.pyi

@@ -13,11 +13,11 @@ class BaseTzInfo(datetime.tzinfo):
     @abstractmethod
     def normalize(self, dt: datetime.datetime) -> datetime.datetime: ...
     @abstractmethod
-    def tzname(self, dt: datetime.datetime | None) -> str: ...
+    def tzname(self, dt: datetime.datetime | None, /) -> str: ...
     @abstractmethod
-    def utcoffset(self, dt: datetime.datetime | None) -> datetime.timedelta | None: ...
+    def utcoffset(self, dt: datetime.datetime | None, /) -> datetime.timedelta | None: ...
     @abstractmethod
-    def dst(self, dt: datetime.datetime | None) -> datetime.timedelta | None: ...
+    def dst(self, dt: datetime.datetime | None, /) -> datetime.timedelta | None: ...
 
 class StaticTzInfo(BaseTzInfo):
     def fromutc(self, dt: datetime.datetime) -> datetime.datetime: ...