fix freqstr in BaseOffset being a property (#430)

* fix freqstr in BaseOffset being a property

* add test for DateOffset freqstr

Author: jkittner

pandas-stubs/_libs/tslibs/offsets.pyi

@@ -79,6 +79,7 @@ class BaseOffset:
     def name(self) -> str: ...
     @property
     def rule_code(self) -> str: ...
+    @property
     def freqstr(self) -> str: ...
     def apply_index(self, dtindex: DatetimeIndex) -> DatetimeIndex: ...
     def rollback(self, dt: datetime) -> datetime: ...

tests/test_timefuncs.py

@@ -18,6 +18,7 @@
 
 from pandas._libs import NaTType
 from pandas._libs.tslibs import BaseOffset
+from pandas._libs.tslibs.offsets import DateOffset
 
 if TYPE_CHECKING:
     from pandas._typing import FulldatetimeDict
@@ -1029,3 +1030,8 @@ def test_timedelta_range() -> None:
         ),
         pd.TimedeltaIndex,
     )
+
+
+def test_dateoffset_freqstr() -> None:
+    offset = DateOffset(minutes=10)
+    check(assert_type(offset.freqstr, str), str)