Fix unsupported operands in .custom-format.py

Python 3.11 complains that int and str are unsupported operand types for
operator +.

.custom-format.py

@@ -40,7 +40,7 @@ def check_clang_format_pip_version():
     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