Don't import Self in Python < 3.11

Author: oscarbenjamin

src/flint/typing.py

@@ -10,12 +10,14 @@
     Iterator,
     Iterable,
     Any,
-    Self,
     Mapping,
     Sequence,
     overload,
 )
 
+if TYPE_CHECKING:
+    from typing import Self
+
 from .flint_base.flint_base import (
     flint_elem,
     flint_scalar,