qtile migrate - defer libcst import

libcst import can be slow so we only import it when we're actually
executing a `qtile migrate` command.

libqtile/scripts/migrate.py

@@ -33,13 +33,9 @@
 if TYPE_CHECKING:
     from collections.abc import Iterator
 
-BACKUP_SUFFIX = ".migrate.bak"
-
-
-try:
     import libcst
-except ImportError:
-    pass
+
+BACKUP_SUFFIX = ".migrate.bak"
 
 
 class AbortMigration(Exception):
@@ -50,6 +46,21 @@ class SkipFile(Exception):
     pass
 
 
+def needs_libcst(func):
+    def _wrapper(*args, **kwargs):
+        if "libcst" not in sys.modules:
+            try:
+                global libcst
+                libcst = __import__("libcst", globals(), locals())
+            except ImportError:
+                print("libcst is needed for 'qtile migrate' commands.")
+                print("Please install it and try again.")
+                sys.exit(1)
+        func(*args, **kwargs)
+
+    return _wrapper
+
+
 def version_tuple(value: str) -> tuple[int, ...]:
     try:
         val = tuple(int(x) for x in value.split("."))
@@ -72,16 +83,12 @@ class QtileMigrate:
     without needing to pass them around all the time.
     """
 
+    @needs_libcst
     def __call__(self, args: argparse.Namespace) -> None:
         """
         This is called by ArgParse when we run `qtile migrate`. The parsed options are
         passed as an argument.
         """
-        if "libcst" not in sys.modules:
-            print("libcst can't be found. Unable to migrate config file.")
-            print("Please install it and try again.")
-            sys.exit(1)
-
         self.args = args
         self.filter_migrations()