Pin PySide6 version (#487)

CI is currently failing thanks to a backwards incompatible change in
PySide6 6.4.0, which affects Pyface. We'll pin the PySide6 version until
Pyface has a chance to adapt to the PySide6 changes.

Fixes #486.

(cherry picked from commit 5cddb45)

setup.py

@@ -331,11 +331,13 @@ def get_long_description():
                 # Avoid https://bugreports.qt.io/browse/PYSIDE-1797, which
                 # causes some versions of PySide6 to be unimportable on Python
                 # 3.6 and 3.7.
+                # PySide6 6.4.0 has incompatible changes that Pyface hasn't
+                # yet adapted to. xref: enthought/pyface#1163
                 (
-                    "pyside6!=6.2.2,!=6.2.2.1,!=6.2.3,!=6.2.4,!=6.3.0; "
+                    "pyside6!=6.2.2,!=6.2.2.1,!=6.2.3,!=6.2.4,!=6.3.0,<6.4.0; "
                     + "python_version<'3.8'"
                 ),
-                "pyside6; python_version>='3.8'",
+                "pyside6<6.4.0; python_version>='3.8'",
             ],
         },
         license="BSD",