use tox to build the documentation

tox.ini

@@ -1,6 +1,6 @@
 # to run for a specific environment, use ``tox -e ENVNAME``
 [tox]
-envlist = py27,py36,py37,py38,py39,py310,pypy3
+envlist = py27,py36,py37,py38,py39,py310,pypy3,docs
 
 [testenv]
 deps =
@@ -19,5 +19,6 @@ commands =
 deps =
     -r {toxinidir}/requirements_docs.txt
 changedir = docs
+allowlist_externals = make
 commands =
     make html