exception: No module named 'sphinx_copybutton'

[fmrtl73]

Describe the bug

Could not import extension sphinx_copybutton (exception: No module named 'sphinx_copybutton')

To Reproduce
I followed the simple instructions to pip install sphinx-copybutton and added the 'sphinx_copybutton' to my extensions:

extensions = [
#    'sphinxmark',
    'sphinxcontrib.spelling',
    'sphinx_rtd_theme',
    'sphinx_copybutton',
]

When I run `tox -e docs` I get the following error:

Extension error:
Could not import extension sphinx_copybutton (exception: No module named 'sphinx_copybutton')

I also tried adding `sphinx_copybutton` to my requirements.txt but that did not help.

**Expected behavior**
It should just work as described in the docs.

**Environment**

- Python Version [e.g. 3.7.1]: 3.9
- Operating System: mac osx 10.15.7

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[chrisjsewell]

Heya, what's in your tox.ini? This creates a separate virtual environment, and so if you haven't specified to install it in there, then it won't find it.