if requirements.txt changes, you must delete your .tox folder

When locally testing the patch [0] which added sphinx_bootstrap_theme to the requirements.txt, I got the following error:

> spectre:docs ckd$ tox -edocs
> docs installed: alabaster==0.7.8,Babel==2.3.4,docutils==0.12,imagesize==0.7.1,Jinja2==2.8,MarkupSafe==0.23,Pygments==2.1.3,pytz==2016.4,robotframework==3.0,six==1.10.0,snowballstemmer==1.2.1,Sphinx==1.4.4
> docs runtests: PYTHONHASHSEED='1128728617'
> docs runtests: commands[0] | sphinx-build -b html -n -d /Users/ckd/git-reps/docs/.tox/docs/tmp/doctrees ./docs/ /Users/ckd/git-reps/docs/docs/_build/html
> Running Sphinx v1.4.4
>
> Exception occurred:
> File "conf.py", line 18, in <module>
> ImportError: No module named sphinx_bootstrap_theme
> The full traceback has been saved in /var/folders/lq/sr8059kj2ql0n2flqtqr40940000gn/T/sphinx-err-l4sZkq.log, if you want to report the issue to the developers.
> Please also report this if it was a user error, so that a better error message can be provided next time.
> A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
> ERROR: InvocationError: '/Users/ckd/git-reps/docs/.tox/docs/bin/sphinx-build -b html -n -d /Users/ckd/git-reps/docs/.tox/docs/tmp/doctrees ./docs/ /Users/ckd/git-reps/docs/docs/_build/html'
> _________________________________________________________________________________________________ summary __________________________________________________________________________________________________
> ERROR: docs: commands failed

Deleting the .tox directory and running tox -edocs again fixed it, but this seems like a weird failure case that tox should catch.

[0] https://git.opendaylight.org/gerrit/#/c/41370/