kpi: Fix NA(dtype) to return object with the same dtype

We were not caring about that and so previously e.g. NA(np.int16) was
giving int instead of np.int16 .

Fix it.

kpi.py

 # -*- coding: utf-8 -*-
-# Copyright (C) 2022  Nexedi SA and Contributors.
+# Copyright (C) 2022-2023  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -634,15 +634,19 @@ def _newscalar(typ, dtype):
 
 # NA returns "Not Available" value for dtype.
 def NA(dtype):
+    typ = dtype.type
     # float
-    if issubclass(dtype.type, np.floating):
-        return np.nan
+    if issubclass(typ, np.floating):
+        na = np.nan
     # int: NA is min value
-    if issubclass(dtype.type, np.signedinteger):
-        return np.iinfo(dtype.type).min
+    elif issubclass(typ, np.signedinteger):
+        na = np.iinfo(typ).min
 
+    else:
         raise AssertionError("NA not defined for dtype %s" % (dtype,))
 
+    return typ(na)  # return the same type as dtype has, e.g. np.int32, not int
+
 
 # isNA returns whether value represent NA.
 # value must be numpy scalar.

kpi_test.py

 # -*- coding: utf-8 -*-
-# Copyright (C) 2022  Nexedi SA and Contributors.
+# Copyright (C) 2022-2023  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -422,7 +422,10 @@ def test_Calc_erab_accessibility():
 
 def test_NA():
     def _(typ):
-        return NA(typ(0).dtype)
+        na = NA(typ(0).dtype)
+        assert type(na) is typ
+        assert isNA(na)
+        return na
 
     assert np.isnan( _(np.float16) )
     assert np.isnan( _(np.float32) )