make dtype to be ignored in constructors if data is Series or DataFrame

The note in the docs wasn't sufficient: the dtype kwarg was ignored in
certain cases. Set it to None if creating Series from another Series and
the same for DataFrames. See GH #59060.

Author: taranarmo

pandas/core/frame.py

@@ -705,6 +705,7 @@ def __init__(
         if isinstance(data, DataFrame):
             data = data._mgr
             allow_mgr = True
+            dtype = None
             if not copy:
                 # if not copying data, ensure to still return a shallow copy
                 # to avoid the result sharing the same Manager

pandas/core/series.py

@@ -445,6 +445,7 @@ def __init__(
                     "compound dtype.  Use DataFrame instead."
                 )
         elif isinstance(data, Series):
+            dtype = None
             if index is None:
                 index = data.index
                 data = data._mgr.copy(deep=False)