Merge pull request matplotlib#15693 from anntzer/pyside2

Also test PySide2 on CI.

Author: dstansby

.travis.yml

@@ -143,6 +143,10 @@ install:
       python -c 'import PyQt5.QtCore' &&
       echo 'PyQt5 is available' ||
       echo 'PyQt5 is not available'
+    python -mpip install --upgrade pyside2 &&
+      python -c 'import PySide2.QtCore' &&
+      echo 'PySide2 is available' ||
+      echo 'PySide2 is not available'
     python -mpip install --upgrade \
       -f https://extras.wxpython.org/wxPython4/extras/linux/gtk3/ubuntu-14.04 \
       wxPython &&

lib/matplotlib/tests/test_backends_interactive.py

@@ -23,18 +23,23 @@ def _get_testable_interactive_backends():
             (["cairo", "gi"], "gtk3cairo"),
             (["PyQt5"], "qt5agg"),
             (["PyQt5", "cairocffi"], "qt5cairo"),
+            (["PySide2"], "qt5agg"),
+            (["PySide2", "cairocffi"], "qt5cairo"),
             (["tkinter"], "tkagg"),
             (["wx"], "wx"),
             (["wx"], "wxagg"),
     ]:
         reason = None
+        missing = [dep for dep in deps if not importlib.util.find_spec(dep)]
         if not os.environ.get("DISPLAY"):
-            reason = "No $DISPLAY"
-        elif any(importlib.util.find_spec(dep) is None for dep in deps):
-            reason = "Missing dependency"
+            reason = "$DISPLAY is unset"
+        elif missing:
+            reason = "{} cannot be imported".format(", ".join(missing))
         if reason:
             backend = pytest.param(
-                backend, marks=pytest.mark.skip(reason=reason))
+                backend,
+                marks=pytest.mark.skip(
+                    reason=f"Skipping {backend} because {reason}"))
         backends.append(backend)
     return backends