to_clipboard() example in docs contains an exception
https://pandas.pydata.org/pandas-docs/version/0.23.4/io.html#clipboard

The code example for the to/from_clipboard() functions contains the following exception in the docs:
```
---------------------------------------------------------------------------
PyperclipException                        Traceback (most recent call last)
<ipython-input-315-d9e762ebf7d3> in <module>()
----> 1 df.to_clipboard()

/pandas/pandas/core/generic.py in to_clipboard(self, excel, sep, **kwargs)
   2247         """
   2248         from pandas.io import clipboards
-> 2249         clipboards.to_clipboard(self, excel=excel, sep=sep, **kwargs)
   2250 
   2251     def to_xarray(self):

/pandas/pandas/io/clipboards.py in to_clipboard(obj, excel, sep, **kwargs)
    124             if PY2:
    125                 text = text.decode('utf-8')
--> 126             clipboard_set(text)
    127             return
    128         except TypeError:

/pandas/pandas/io/clipboard/clipboards.py in __call__(self, *args, **kwargs)
    132 
    133         def __call__(self, *args, **kwargs):
--> 134             raise PyperclipException(EXCEPT_MSG)
    135 
    136         if PY2:

PyperclipException: 
    Pyperclip could not find a copy/paste mechanism for your system.
    For more information, please visit https://pyperclip.readthedocs.org 
```
This issue is not present in the previous version's docs: https://pandas.pydata.org/pandas-docs/version/0.23.3/io.html#clipboard

Probably just a setup issue for whatever system is building the documentation.