DOC: clipboard library missing in doc env on travis
The doc builds sees the following failure: 

```
PyperclipException: 
    Pyperclip could not find a copy/paste mechanism for your system.
    For more information, please visit https://pyperclip.readthedocs.org 
```

which is happening since the refactor of the CI envs in https://github.com/pandas-dev/pandas/pull/24632. In unifying things we might have by accident removed some clipboard related things for the doc build.

cc @saurav2608 if you want to take a look at this, always welcome

