Turn off flake8 completely for ubench/

To suppress these new errors (apparently after a flake8 upgrade):

```
flake8...................................................................Failed
- hook id: flake8
- exit code: 1

ubench/holder_comparison.py:96:38: B023 Function definition does not bind loop variable 'nb1'.
                    assert int(round(nb1.sum())) == data_size
                                     ^
ubench/holder_comparison.py:96:53: B023 Function definition does not bind loop variable 'data_size'.
                    assert int(round(nb1.sum())) == data_size
                                                    ^
ubench/holder_comparison.py:99:25: B023 Function definition does not bind loop variable 'nb1'.
                        nb1.sum()
                        ^
ubench/holder_comparison.py:103:28: B023 Function definition does not bind loop variable 'nb1'.
                    assert nb1.add(nb2) == data_size
                           ^
ubench/holder_comparison.py:103:36: B023 Function definition does not bind loop variable 'nb2'.
                    assert nb1.add(nb2) == data_size
                                   ^
ubench/holder_comparison.py:103:44: B023 Function definition does not bind loop variable 'data_size'.
                    assert nb1.add(nb2) == data_size
                                           ^
ubench/holder_comparison.py:106:25: B023 Function definition does not bind loop variable 'nb1'.
                        nb1.add(nb2)
                        ^
ubench/holder_comparison.py:106:33: B023 Function definition does not bind loop variable 'nb2'.
                        nb1.add(nb2)
                                ^
ubench/holder_comparison_extract_sheet_data.py:21:16: B023 Function definition does not bind loop variable 'header_row'.
            if header_row:
               ^
ubench/holder_comparison_extract_sheet_data.py:22:20: B023 Function definition does not bind loop variable 'header'.
                if header is None:  # type: ignore[unreachable]
                   ^
ubench/holder_comparison_extract_sheet_data.py:23:36: B023 Function definition does not bind loop variable 'header_row'.
                    print(",".join(header_row))
                                   ^
ubench/holder_comparison_extract_sheet_data.py:25:28: B023 Function definition does not bind loop variable 'header'.
                    assert header == header_row
                           ^
ubench/holder_comparison_extract_sheet_data.py:25:38: B023 Function definition does not bind loop variable 'header_row'.
                    assert header == header_row
                                     ^
ubench/holder_comparison_extract_sheet_data.py:26:16: B023 Function definition does not bind loop variable 'data_row'.
            if data_row is not None:
               ^
ubench/holder_comparison_extract_sheet_data.py:27:32: B023 Function definition does not bind loop variable 'data_row'.
                print(",".join(data_row))  # type: ignore[unreachable]
                               ^
ubench/holder_comparison_extract_sheet_data.py:28:17: B023 Function definition does not bind loop variable 'data_row_buffer'.
                data_row_buffer.append(data_row)
                ^
ubench/holder_comparison_extract_sheet_data.py:28:40: B023 Function definition does not bind loop variable 'data_row'.
                data_row_buffer.append(data_row)
                                       ^
ubench/holder_comparison_extract_sheet_data.py:29:20: B023 Function definition does not bind loop variable 'header_row'.
            return header_row
                   ^
```