BLD: Error while installing requirements-dev.txt
While running the command on a fresh virtual env (using pip), I got the error messages: 

```
ERROR: Could not find a version that satisfies the requirement python==3.7 (from -r requirements-dev.txt (line 2)) (from versions: none)
ERROR: No matching distribution found for python==3.7 (from -r requirements-dev.txt (line 2))
```

I can't run ```pd.show_versions()```.
So here's what I've got:

 - pip -19.3.1 
 - setuptools - 41.6.0


SS of the error:
![no_python_3_7](https://user-images.githubusercontent.com/50263213/68335787-ce7c8800-00e5-11ea-9c13-65ab9a2f82d9.png)


Full text:
```
(venv-pandas) [bummy@null pandas-MomIsBestFriend]$ python -m pip install -r requirements-dev.txt 
Collecting git+https://github.com/pandas-dev/pandas-sphinx-theme.git@master (from -r requirements-dev.txt (line 68))
  Cloning https://github.com/pandas-dev/pandas-sphinx-theme.git (to revision master) to /tmp/pip-req-build-_jc5c_b6
  Running command git clone -q https://github.com/pandas-dev/pandas-sphinx-theme.git /tmp/pip-req-build-_jc5c_b6
Collecting numpy>=1.15
  Using cached https://files.pythonhosted.org/packages/00/4a/e34fce8f18c0e052c2b49f1b3713469d855f7662d58ae2b82a88341e865b/numpy-1.17.3-cp37-cp37m-manylinux1_x86_64.whl
ERROR: Could not find a version that satisfies the requirement python==3.7 (from -r requirements-dev.txt (line 2)) (from versions: none)
ERROR: No matching distribution found for python==3.7 (from -r requirements-dev.txt (line 2))
(venv-pandas) [bummy@null pandas-MomIsBestFriend]$ 
```
