Fix ruff linter errors: add migrate to __all__, remove unused import

Author: claude

src/datajoint/__init__.py

@@ -50,6 +50,7 @@
     "list_types",
     "AttributeAdapter",  # Deprecated, use AttributeType
     "errors",
+    "migrate",
     "DataJointError",
     "key",
     "key_hash",

src/datajoint/migrate.py

@@ -15,7 +15,6 @@
 from .errors import DataJointError
 
 if TYPE_CHECKING:
-    from .connection import Connection
     from .schemas import Schema
 
 logger = logging.getLogger(__name__.split(".")[0])