Strange behaviour of `delim_whitespace` in `pd.read_table`
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
import pandas as pd
# The error persists with non-empty files
! touch test_file
pd.read_table('./test_file', delim_whitespace=True)
```

#### Problem description

```

~/.local/lib/python3.6/site-packages/pandas/io/parsers.py in read_csv(filepath_or_buffer, sep, delimiter, header, names, index_col, usecols, squeeze, prefix, mangle_dupe_cols, dtype, engine, converters, true_values, false_values, skipinitialspace, skiprows, skipfooter, nrows, na_values, keep_default_na, na_filter, verbose, skip_blank_lines, parse_dates, infer_datetime_format, keep_date_col, date_parser, dayfirst, cache_dates, iterator, chunksize, compression, thousands, decimal, lineterminator, quotechar, quoting, doublequote, escapechar, comment, encoding, dialect, error_bad_lines, warn_bad_lines, delim_whitespace, low_memory, memory_map, float_precision)
    623     if delim_whitespace and delimiter != default_sep:
    624         raise ValueError(
--> 625             "Specified a delimiter with both sep and "
    626             "delim_whitespace=True; you can only specify one."
    627         )

ValueError: Specified a delimiter with both sep and delim_whitespace=True; you can only specify one.
```
#### Expected Output

Parsed dataframe.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : 2a7d3326dee660824a8433ffd01065f8ac37f7d6
python           : 3.6.10.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.12.14-lp151.28.67-default
Version          : #1 SMP Fri Sep 4 15:23:21 UTC 2020 (2c5a14f)
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.2
numpy            : 1.17.3
pytz             : 2018.5
dateutil         : 2.7.3
pip              : 19.3.1
setuptools       : 40.5.0
Cython           : 0.27.3
pytest           : None
hypothesis       : None
sphinx           : 1.7.6
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.0.0
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10.1
IPython          : 7.4.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 2.0.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.2.0
sqlalchemy       : 1.2.14
tables           : None
tabulate         : 0.8.5
xarray           : None
xlrd             : None
xlwt             : None
numba            : 0.48.0


</details>
