GH1099 Remove how arg from Periodindex.astype (#1100)

Author: loicdiridollou

pandas-stubs/core/indexes/period.pyi

@@ -53,7 +53,7 @@ class PeriodIndex(DatetimeIndexOpsMixin[pd.Period], PeriodIndexFieldOps):
     def __array__(self, dtype=...) -> np.ndarray: ...
     def __array_wrap__(self, result, context=...): ...
     def asof_locs(self, where, mask): ...
-    def astype(self, dtype, copy: bool = ..., how: str = ...): ...
+    def astype(self, dtype, copy: bool = ...): ...
     def searchsorted(self, value, side: str = ..., sorter=...): ...
     @property
     def is_full(self) -> bool: ...