TST: Fix tests with older versions of ipython

I only went back as far as 7.0.0, as due to matplotlib#16263, we probably don't
want to be supporting all the way back to IPython 1.

.github/workflows/tests.yml

@@ -60,7 +60,7 @@ jobs:
           - os: ubuntu-20.04
             python-version: 3.9
             # One CI run tests ipython/matplotlib-inline before backend mapping moved to mpl
-            extra-requirements: '-r requirements/testing/extra.txt "ipython<8.24" "matplotlib-inline<0.1.7"'
+            extra-requirements: '-r requirements/testing/extra.txt "ipython==7.19" "matplotlib-inline<0.1.7"'
             CFLAGS: "-fno-lto"  # Ensure that disabling LTO works.
             # https://github.com/matplotlib/matplotlib/pull/26052#issuecomment-1574595954
             # https://www.riverbankcomputing.com/pipermail/pyqt/2023-November/045606.html

lib/matplotlib/testing/__init__.py

@@ -179,11 +179,7 @@ def _has_tex_package(package):
         return False
 
 
-def ipython_in_subprocess(
-    requested_backend_or_gui_framework,
-    expected_backend_old_ipython,  # IPython < 8.24
-    expected_backend_new_ipython,  # IPython >= 8.24
-):
+def ipython_in_subprocess(requested_backend_or_gui_framework, all_expected_backends):
     import pytest
     IPython = pytest.importorskip("IPython")
 
@@ -194,12 +190,12 @@ def ipython_in_subprocess(
             requested_backend_or_gui_framework == "osx"):
         pytest.skip("Bug using macosx backend in IPython 8.24.0 fixed in 8.24.1")
 
-    if IPython.version_info[:2] >= (8, 24):
-        expected_backend = expected_backend_new_ipython
-    else:
-        # This code can be removed when Python 3.12, the latest version supported by
-        # IPython < 8.24, reaches end-of-life in late 2028.
-        expected_backend = expected_backend_old_ipython
+    # This code can be removed when Python 3.12, the latest version supported
+    # by IPython < 8.24, reaches end-of-life in late 2028.
+    for min_version, backend in all_expected_backends.items():
+        if IPython.version_info[:2] >= min_version:
+            expected_backend = backend
+            break
 
     code = ("import matplotlib as mpl, matplotlib.pyplot as plt;"
             "fig, ax=plt.subplots(); ax.plot([1, 3, 2]); mpl.get_backend()")

lib/matplotlib/testing/__init__.pyi

@@ -49,6 +49,5 @@ def _check_for_pgf(texsystem: str) -> bool: ...
 def _has_tex_package(package: str) -> bool: ...
 def ipython_in_subprocess(
     requested_backend_or_gui_framework: str,
-    expected_backend_old_ipython: str,
-    expected_backend_new_ipython: str,
+    all_expected_backends: dict[tuple[int, int], str],
 ) -> None: ...

lib/matplotlib/tests/test_backend_macosx.py

@@ -48,4 +48,4 @@ def new_choose_save_file(title, directory, filename):
 
 def test_ipython():
     from matplotlib.testing import ipython_in_subprocess
-    ipython_in_subprocess("osx", "MacOSX", "macosx")
+    ipython_in_subprocess("osx", {(8, 24): "macosx", (7, 0): "MacOSX"})

lib/matplotlib/tests/test_backend_qt.py

@@ -378,4 +378,4 @@ def custom_handler(signum, frame):
 
 def test_ipython():
     from matplotlib.testing import ipython_in_subprocess
-    ipython_in_subprocess("qt", "QtAgg", "qtagg")
+    ipython_in_subprocess("qt", {(8, 24): "qtagg", (8, 15): "QtAgg", (7, 0): "Qt5Agg"})