fix: importing QmatchateaBackend inside proper condition in the __init__ file

src/qibotn/backends/__init__.py

@@ -4,11 +4,8 @@ from qibo.config import raise_error
 
 from qibotn.backends.abstract import QibotnBackend
 from qibotn.backends.cutensornet import CuTensorNet  # pylint: disable=E0401
-from qibotn.backends.qmatchatea import QMatchaTeaBackend  # pylint: disable=E0401
 from qibotn.backends.quimb import QuimbBackend  # pylint: disable=E0401
 
-QibotnBackend = Union[CuTensorNet, QuimbBackend, QMatchaTeaBackend]
-
 PLATFORMS = ("cutensornet", "qutensornet", "qmatchatea")
 
 
@@ -31,6 +28,8 @@ class MetaBackend:
         elif platform == "qutensornet":  # pragma: no cover
             return QuimbBackend(runcard)
         elif platform == "qmatchatea":  # pragma: no cover
+            from qibotn.backends.qmatchatea import QMatchaTeaBackend
+
             return QMatchaTeaBackend()
         else:
             raise_error(