Fix typing runtime issue

modmail/utils/string.py

@@ -26,6 +26,8 @@
 # OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+from __future__ import annotations
+
 import typing as t
 
 if t.TYPE_CHECKING:
@@ -48,6 +50,6 @@ def ordinal(number: int) -> str:
     return f"{number:,}th"
 
 
-def possessive(user: Member | User) -> str:
+def possessive(user: "Member" | "User") -> str:
     name = getattr(user, "display_name", user.username)
     return f"{name}'{'s' if not name.endswith('s') else ''}"