Cannot install pandas 0.24.2 from source with Python 2.7 into an environment without NumPy
#### Code Sample, a copy-pastable example if possible

```bash
RuntimeError: Python version >= 3.5 required.
```
#### Problem description

Under this docs: https://pandas.pydata.org/pandas-docs/stable/whatsnew/v0.24.0.html

0.24.x is supposed to be backward compatible to Python 2.

```
The 0.24.x series of releases will be the last to support Python 2. Future feature releases will support Python 3 only. See Plan for dropping Python 2.7 for more details
```

#### Expected Output

0.24.2 should still work on Python 2.
