Don't force matplotlib backend names to be lowercase (#14475)

This is something I missed as part of moving the backend resolution to
Matplotlib. Since version 3.9.0 Matplotlib handles the case-sensitivity
of backend names, making them all lowercase for internal use. But since
matplotlib/matplotlib#28473 uppercase letters are allowed in backend
names of the form `module://soMe_moDule.sOmE_NAme`. Hence we need to not
force backend names to be lowercase in IPython when passing them to
Matplotlib. It is just a one line change that happens to make the code
simpler, plus a new test.

The Matplotlib change will be released in 3.9.1 which is due shortly,
possibly today. The new test checks the Matplotlib version to know
whether uppercase backend module names are allowed or not.

IPython/core/magics/pylab.py

@@ -100,7 +100,7 b' class PylabMagics(Magics):'
                 % _list_matplotlib_backends_and_gui_loops()
             )
         else:
-            gui, backend = self.shell.enable_matplotlib(args.gui.lower() if isinstance(args.gui, str) else args.gui)
+            gui, backend = self.shell.enable_matplotlib(args.gui)
             self._show_matplotlib_backend(args.gui, backend)
 
     @skip_doctest

IPython/core/tests/test_pylabtools.py

@@ -258,6 +258,29 b' class TestPylabSwitch(object):'
         assert gui == "qt"
         assert s.pylab_gui_select == "qt"
 
+    @dec.skipif(not pt._matplotlib_manages_backends())
+    def test_backend_module_name_case_sensitive(self):
+        # Matplotlib backend names are case insensitive unless explicitly specified using
+        # "module://some_module.some_name" syntax which are case sensitive for mpl >= 3.9.1
+        all_lowercase = "module://matplotlib_inline.backend_inline"
+        some_uppercase = "module://matplotlib_inline.Backend_inline"
+        mpl3_9_1 = matplotlib.__version_info__ >= (3, 9, 1)
+
+        s = self.Shell()
+        s.enable_matplotlib(all_lowercase)
+        if mpl3_9_1:
+            with pytest.raises(RuntimeError):
+                s.enable_matplotlib(some_uppercase)
+        else:
+            s.enable_matplotlib(some_uppercase)
+
+        s.run_line_magic("matplotlib", all_lowercase)
+        if mpl3_9_1:
+            with pytest.raises(RuntimeError):
+                s.run_line_magic("matplotlib", some_uppercase)
+        else:
+            s.run_line_magic("matplotlib", some_uppercase)
+
 
 def test_no_gui_backends():
     for k in ['agg', 'svg', 'pdf', 'ps']: