feat: add cython defs for Variant class (#74)

src/dbus_fast/signature.pxd

@@ -14,3 +14,10 @@ cdef class SignatureTree:
 
     cdef public str signature
     cdef public list types
+
+
+cdef class Variant:
+
+    cdef public object type
+    cdef public str signature
+    cdef public object value

src/dbus_fast/signature.py

@@ -403,7 +403,8 @@ class Variant:
         signature: Union[str, SignatureTree, SignatureType],
         value: Any,
         verify: bool = True,
-    ):
+    ) -> None:
+        """Init a new Variant."""
         if type(signature) is SignatureTree:
             signature_tree = signature
         elif type(signature) is SignatureType: