BUG: read_table cannot handle multi-character separators in memory_map mode
- [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.

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

---

#### Code Sample

```python
import pandas
dataframe = pandas.read_table('file.txt',
                              header=None,
                              sep=' - ',
                              names=['key', 'value'],
                              memory_map=True)
```

#### Data Sample
```
key1 - value1
key2 - value2
key3 - value3
```

#### Problem description

I'm getting an exception: 
> **Exception has occurred: TypeError**
cannot use a string pattern on a bytes-like object

from this line:
https://github.com/pandas-dev/pandas/blob/14eda586582513c68f32f0a1f00ecfe8d6c7f8f3/pandas/io/parsers.py#L2472
since `f.readline()` returns a byte-string.

And if I change separator to `sep=b' - '`, a similar exception:
> **Exception has occurred: TypeError**
cannot use a bytes pattern on a string-like object

gets raised from the following line:
https://github.com/pandas-dev/pandas/blob/14eda586582513c68f32f0a1f00ecfe8d6c7f8f3/pandas/io/parsers.py#L2475
since `for line in f:` yields normal strings.

#### Expected Output

Exception should not be raised either with `sep=' - '` or with `sep=b' - '`.
Not sure which one is the correct, although I see no issues with `sep=' - '` when `memory_map=True` is not set.

#### Workaround
Changing lines
https://github.com/pandas-dev/pandas/blob/14eda586582513c68f32f0a1f00ecfe8d6c7f8f3/pandas/io/parsers.py#L2474-L2475
to
```python
while True:
    line = f.readline()
    if not line:
        break
    yield pat.split(line.strip())
```
fixes `read_table` for byte-string separators (e.g. `sep=b' - '`).

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.5.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 63 Stepping 2, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : None.None

pandas           : 1.0.4
numpy            : 1.18.5
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 41.2.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : 1.3.17
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None

</details>
