Fix unsupported operands in .custom-format.py

Python 3.11 complains that int and str are unsupported operand types for
operator +.
diff --git a/.custom-format.py b/.custom-format.py
index d07c26d..1295ce5 100755
--- a/.custom-format.py
+++ b/.custom-format.py
@@ -40,7 +40,7 @@
     if importlib.util.find_spec('clang_format'):
         # Check if the installed version is the expected LLVM version
         if importlib.metadata.version('clang-format')\
-                .startswith(CURRENT_LLVM+'.'):
+                .startswith(str(CURRENT_LLVM)+'.'):
             return True
         else:
             # Return False, because the clang-format version does not match