EHN: Implement closed=left|right|neither in DataFrame.rolling for fixed windows
- [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.

---

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

```python
import pandas as pd

data = [0, 1, 1, 0, 0, 1, 0, 1]
df = pd.DataFrame({'binary_col': data}, index=pd.date_range(start='2020-01-01', freq="min", periods=len(data)))

rolling = df.rolling(window=len(df), closed='left', min_periods=1)
rolling.mean()
```

#### Problem description

The rolling window ignores the closed='left' parameter. Since it was supposed to be closed only on the left side, the current right side of the window should not be used on the calculations. But it turns out it is.

#### Original dataframe

| index               | binary_col |
|---------------------|------------|
| 2020-01-01 00:00:00 |          0 |
| 2020-01-01 00:01:00 |          1 |
| 2020-01-01 00:02:00 |          1 |
| 2020-01-01 00:03:00 |          0 |
| 2020-01-01 00:04:00 |          0 |
| 2020-01-01 00:05:00 |          1 |
| 2020-01-01 00:06:00 |          0 |
| 2020-01-01 00:07:00 |          1 |

#### Actual output

| index               | binary_col |
|---------------------|------------|
| 2020-01-01 00:00:00 |   0.000000 |
| 2020-01-01 00:01:00 |   0.500000 |
| 2020-01-01 00:02:00 |   0.666667 |
| 2020-01-01 00:03:00 |   0.500000 |
| 2020-01-01 00:04:00 |   0.400000 |
| 2020-01-01 00:05:00 |   0.500000 |
| 2020-01-01 00:06:00 |   0.428571 |
| 2020-01-01 00:07:00 |   0.500000 |

#### Expected Output

| index               | binary_col |
|---------------------|------------|
| 2020-01-01 00:00:00 |   NaN |
| 2020-01-01 00:01:00 |   0.000000 |
| 2020-01-01 00:02:00 |   0.500000 |
| 2020-01-01 00:03:00 |  0.666667 |
| 2020-01-01 00:04:00 |   0.500000 |
| 2020-01-01 00:05:00 |   0.400000 |
| 2020-01-01 00:06:00 |  0.500000  |
| 2020-01-01 00:07:00 |   0.428571 |

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

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.7.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-70-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : pt_BR.UTF-8

pandas           : 1.0.3
numpy            : 1.18.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.2.0.post20200511
Cython           : None
pytest           : 5.4.2
hypothesis       : None
sphinx           : 3.0.3

</details>
