Revert "Remove dtype lists"

This reverts commit 6e39a2e.

skimage/_shared/dtype.py

@@ -1,5 +1,30 @@
 import numpy as np
 
+# Define classes of supported dtypes and Python scalar types
+# Variables ending in `_dtypes` only contain numpy.dtypes of the respective
+# class; variables ending in `_types` additionally include Python scalar types.
+signed_integer_dtypes = {np.int8, np.int16, np.int32, np.int64}
+signed_integer_types = signed_integer_dtypes | {int}
+
+unsigned_integer_dtypes = {np.uint8, np.uint16, np.uint32, np.uint64}
+
+integer_dtypes = signed_integer_dtypes | unsigned_integer_dtypes
+integer_types = signed_integer_types | unsigned_integer_dtypes
+
+floating_dtypes = {np.float16, np.float32, np.float64}
+floating_types = floating_dtypes | {float}
+
+complex_dtypes = {np.complex64, np.complex128}
+complex_types = complex_dtypes | {complex}
+
+inexact_dtypes = floating_dtypes | complex_dtypes
+inexact_types = floating_types | complex_types
+
+bool_types = {np.dtype(bool), bool}
+
+numeric_dtypes = integer_dtypes | inexact_dtypes | {np.bool_}
+numeric_types = integer_types | inexact_types | bool_types
+
 
 def numeric_dtype_min_max(dtype):
     """Return minimum and maximum representable value for a given dtype.