[utils/check_copies.py] fix DeprecationWarning
in `tests/test_utils_check_copies.py` I was getting intermittently:
```
utils/check_copies.py:52
  /mnt/nvme1/code/transformers-comet/utils/check_copies.py:52: DeprecationWarning: invalid escape sequence \s
    while line_index < len(lines) and re.search(f"^{indent}(class|def)\s+{name}", lines[line_index]) is None:
```
So this should fix it. Not sure why it wasn't showing up all the time.

@sgugger 
Thanks!