TYP: Improve infer_freq

Author: bashtage

pandas-stubs/tseries/frequencies.pyi

@@ -1,6 +1,12 @@
+from pandas import (
+    DatetimeIndex,
+    Series,
+    TimedeltaIndex,
+)
+
 from pandas.tseries.offsets import DateOffset as DateOffset
 
 def get_period_alias(offset_str: str) -> str | None: ...
 def to_offset(freq) -> DateOffset | None: ...
 def get_offset(name: str) -> DateOffset: ...
-def infer_freq(index) -> str | None: ...
+def infer_freq(index: Series | DatetimeIndex | TimedeltaIndex) -> str | None: ...