Add single quotes around column names if strings

[gforsyth]

• Tests added / passed
• Passes black dask / flake8 dask

Fixes #6470

Adds explicit single quotes around the column names in bad_dtypes so that it's easier to distinguish between str(int) column names and int column names.

Traceback (most recent call last):
  File "metadata_mismatch.py", line 11, in <module>
    df.compute()
  File "/Users/gil/github.com/dask/dask/dask/base.py", line 167, in compute
    (result,) = compute(self, traverse=False, **kwargs)
  File "/Users/gil/github.com/dask/dask/dask/base.py", line 447, in compute
    results = schedule(dsk, keys, **kwargs)
  File "/Users/gil/github.com/dask/dask/dask/threaded.py", line 76, in get
    results = get_async(
  File "/Users/gil/github.com/dask/dask/dask/local.py", line 486, in get_async
    raise_exception(exc, tb)
  File "/Users/gil/github.com/dask/dask/dask/local.py", line 316, in reraise
    raise exc
  File "/Users/gil/github.com/dask/dask/dask/local.py", line 222, in execute_task
    result = _execute_task(task, data)
  File "/Users/gil/github.com/dask/dask/dask/core.py", line 121, in _execute_task
    return func(*(_execute_task(a, cache) for a in args))
  File "/Users/gil/github.com/dask/dask/dask/dataframe/utils.py", line 672, in check_meta
    raise ValueError(
ValueError: Metadata mismatch found in `from_delayed`.

Partition type: `pandas.core.frame.DataFrame`
+--------+-------+----------+
| Column | Found | Expected |
+--------+-------+----------+
| '7'    | int64 | -        |
| '8'    | int64 | -        |
| 7      | -     | int64    |
| 8      | -     | int64    |
+--------+-------+----------+

[mrocklin]

Maybe repr(col) instead?

[gforsyth]

yep, that's a better idea.