Trac #33491: fix usage of Matplotlib color maps

Matplotlib used to have a global dictionary of color maps. By now, it is
considered internal API and does not contain newer color maps anymore,
so some of them cannot be used in Sage:

{{{
sage: density_plot(x, (-1,1), (-1,1), cmap='turbo')
/usr/lib/python3.10/site-
packages/sage/repl/rich_output/display_manager.py:608: RichReprWarning:
Exception in _rich_repr_ while displaying object:
Color map turbo not known (type import matplotlib.cm;
matplotlib.cm.datad.keys() for valid names)
}}}

URL: https://trac.sagemath.org/33491
Reported by: gh-mwageringel
Ticket author(s): Markus Wageringel
Reviewer(s): Travis Scrimshaw

src/sage/plot/colors.py

@@ -21,11 +21,11 @@
 For a list of color maps in Sage, evaluate::
 
     sage: sorted(colormaps)
-    ['Accent', 'Blues', 'BrBG', ...]
+    ['Accent', ...]
 
-These are imported from matplotlib's cm_ module.
+These are imported from matplotlib's colormaps_ collection.
 
-.. _cm: http://matplotlib.sourceforge.net/api/cm_api.html
+.. _colormaps: https://matplotlib.org/stable/gallery/color/colormap_reference.html
 """
 
 # ****************************************************************************
@@ -41,10 +41,6 @@
 from colorsys import hsv_to_rgb, hls_to_rgb, rgb_to_hsv, rgb_to_hls
 
 
-# matplotlib color maps, loaded on-demand
-cm = None
-
-
 colors_dict = {
     'automatic'              : '#add8e6',     # 173, 216, 230
     'aliceblue'              : '#f0f8ff',     # 240, 248, 255
@@ -1332,7 +1328,7 @@ def get_cmap(cmap):
     and color names.  For a list of map names, evaluate::
 
         sage: sorted(colormaps)
-        ['Accent', 'Blues', ...]
+        ['Accent', ...]
 
     See :func:`rgbcolor` for valid list/tuple element formats.
 
@@ -1360,25 +1356,31 @@ def get_cmap(cmap):
         sage: get_cmap('jolies')
         Traceback (most recent call last):
         ...
-        RuntimeError: Color map jolies not known (type import matplotlib.cm; matplotlib.cm.datad.keys() for valid names)
+        RuntimeError: Color map jolies not known (type "import matplotlib; list(matplotlib.colormaps.keys())" for valid names)
         sage: get_cmap('mpl')
         Traceback (most recent call last):
         ...
-        RuntimeError: Color map mpl not known (type import matplotlib.cm; matplotlib.cm.datad.keys() for valid names)
+        RuntimeError: Color map mpl not known (type "import matplotlib; list(matplotlib.colormaps.keys())" for valid names)
+
+    TESTS:
+
+    Check that :trac:`33491` is fixed::
+
+        sage: get_cmap('turbo')
+        <matplotlib.colors.ListedColormap object at 0x...>
     """
     # matplotlib color maps
-    global cm
-    if not cm:
-        from matplotlib import cm
     from matplotlib.colors import ListedColormap, Colormap
 
     if isinstance(cmap, Colormap):
         return cmap
 
     elif isinstance(cmap, str):
-        if cmap not in cm.datad:
-            raise RuntimeError("Color map %s not known (type import matplotlib.cm; matplotlib.cm.datad.keys() for valid names)" % cmap)
-        return cm.__dict__[cmap]
+        from matplotlib import colormaps
+        try:
+            return colormaps[cmap]
+        except KeyError:
+            raise RuntimeError('Color map %s not known (type "import matplotlib; list(matplotlib.colormaps.keys())" for valid names)' % cmap)
 
     elif isinstance(cmap, (list, tuple)):
         cmap = [rgbcolor(_) for _ in cmap]
@@ -1424,7 +1426,7 @@ class Colormaps(MutableMapping):
     For a list of map names, evaluate::
 
         sage: sorted(colormaps)
-        ['Accent', 'Blues', ...]
+        ['Accent', ...]
     """
     def __init__(self):
         """
@@ -1453,13 +1455,12 @@ def load_maps(self):
             sage: maps.load_maps()
             sage: len(maps.maps)>60
             True
+            sage: 'viridis' in maps
+            True
         """
-        global cm
-        if not cm:
-            from matplotlib import cm
         if not self.maps:
-            for cmap in cm.datad:
-                self.maps[cmap] = cm.__getattribute__(cmap)
+            from matplotlib import colormaps
+            self.maps.update(colormaps)
 
     def __dir__(self):
         """