diff --git a/cmocean/cm.py b/cmocean/cm.py
index 7ffe563..e70984c 100644
--- a/cmocean/cm.py
+++ b/cmocean/cm.py
@@ -38,8 +38,11 @@ def _register_cmap(cmap, *, name):
     if MPL_VERSION >= Version("3.5"):
         mpl.colormaps.register(cmap, name=name)
     else:
-        # deprecated API
-        mpl.cm.register_cmap(name=name, cmap=cmap)
+        # deprecated API.  `matplotlib.cm` is imported here because importing
+        # `matplotlib` alone does not make the submodule available, so this
+        # branch used to raise an AttributeError.
+        import matplotlib.cm
+        matplotlib.cm.register_cmap(name=name, cmap=cmap)
 
 # add colormaps and reversed to dictionary
 for cmapname in cmapnames:
@@ -53,11 +56,11 @@ def _register_cmap(cmap, *, name):
     rgb_with_alpha = np.zeros((rgb.shape[0],4))
     rgb_with_alpha[:,:3] = rgb
     rgb_with_alpha[:,3]  = 1.  #set alpha channel to 1
-    reg_map = colors.ListedColormap(rgb_with_alpha, N=rgb.shape[0])
+    reg_map = colors.ListedColormap(rgb_with_alpha)
     _register_cmap(reg_map, name=f'cmo.{cmapname}')
 
     # Register the reversed map
-    reg_map_r = colors.ListedColormap(rgb_with_alpha[::-1,:], N=rgb.shape[0])
+    reg_map_r = colors.ListedColormap(rgb_with_alpha[::-1,:])
     _register_cmap(reg_map_r, name=f'cmo.{cmapname}_r')
 
     # Load inverted cmaps
@@ -71,9 +74,9 @@ def _register_cmap(cmap, *, name):
     rgb_with_alpha[:,3]  = 1.  #set alpha channel to 1
 
     # Register inverted cmaps
-    reg_map_i = colors.ListedColormap(rgb_with_alpha, N=rgb_i.shape[0])
+    reg_map_i = colors.ListedColormap(rgb_with_alpha)
     _register_cmap(reg_map_i, name=f'cmo.{cmapname}_i')
-    reg_map_r_i = colors.ListedColormap(rgb_with_alpha[::-1,:], N=rgb_i.shape[0])
+    reg_map_r_i = colors.ListedColormap(rgb_with_alpha[::-1,:])
     _register_cmap(reg_map_r_i, name=f'cmo.{cmapname}_r_i')
 
     # order shouldn't matter
diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..b2fcc25
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,3 @@
+[build-system]
+requires = ["setuptools>=64"]
+build-backend = "setuptools.build_meta"
diff --git a/setup.py b/setup.py
index 95d68bc..5c2c25b 100644
--- a/setup.py
+++ b/setup.py
@@ -4,22 +4,11 @@
 setup.py for cmocean
 
 """
-import sys
 from setuptools import setup # to support "develop" mode
-from setuptools.command.test import test as TestCommand
-
-class PyTest(TestCommand):
-    def finalize_options(self):
-        TestCommand.finalize_options(self)
-        self.verbose = True
-
-    def run_tests(self):
-        import pytest
-        errno = pytest.main(self.test_args)
-        sys.exit(errno)
 
 extras_require={
     'plots':  ["colorspacious", "viscm"],
+    'test':  ["pytest"],
 }
 # # in case I add more later
 # extras_require['complete'] = sorted(set(sum(extras_require.values(), [])))
@@ -44,9 +33,7 @@ def run_tests(self):
     ext_package='cmocean',
     scripts = [],
     keywords = ['colormaps', 'oceanography', 'plotting', 'visualization'],
-    setup_requires=['setuptools'],
     install_requires=['matplotlib', 'numpy', 'packaging'],
-    tests_require=['pytest'],
     python_requires=">=3.8",
     extras_require=extras_require
     )