Add ability to set custom docs-dependencies.

Used for zopefoundation/zope.hookable#30

config/README.rst

@@ -241,13 +241,16 @@ updated. Example:
         ]
     coverage-basepython = "python3.9"
     coverage-command = [
-        "coverage run {envbindir}/test_with_gs []"
+        "coverage run {envbindir}/test_with_gs []",
         ]
     coverage-setenv = [
         "COVERAGE_HOME={toxinidir}",
         ]
     coverage-additional = [
-        "depends = py37,docs"
+        "depends = py37,docs",
+        ]
+    docs-deps = [
+        "urllib3 < 2",
         ]
     use-flake8 = true
 
@@ -483,6 +486,12 @@ coverage-additional
   This option allows to add additional lines below ``[testenv:coverage]`` in
   ``tox.ini``. This option has to be a list of strings.
 
+docs-deps
+  This option allows to add additional install dependencies for
+  ``[testenv:docs]`` in ``tox.ini``. This option has to be a list of strings
+  and is empty by default. Caution: The values set for this option overwrite
+  the ones set in ``[testenv]``.
+
 use-flake8
   Whether to add the ``flake8`` and ``isort`` linting steps to the section
   ``[testenv:lint]``. By default these steps are run. On an older code base it

config/config-package.py

@@ -432,6 +432,7 @@ def tox(self):
             use_flake8 = self.tox_option('use-flake8', default=True)
         else:
             use_flake8 = self.args.use_flake8
+        docs_deps = self.tox_option('docs-deps', default=[])
         self.meta_cfg['tox']['use-flake8'] = use_flake8
         self.copy_with_meta(
             'tox.ini.j2',
@@ -458,7 +459,7 @@ def tox(self):
             with_future_python=self.with_future_python,
             with_pypy=self.with_pypy,
             with_sphinx_doctests=self.with_sphinx_doctests,
-            with_config_type=self.config_type,
+            docs_deps=docs_deps,
         )
 
     def tests_yml(self):

config/default/tox-docs.j2

@@ -3,6 +3,12 @@
 [testenv:docs]
 basepython = python3
 skip_install = false
+{% if docs_deps %}
+deps =
+  {% for line in docs_deps %}
+    %(line)s
+  {% endfor %}
+{% endif %}
 {% if not with_sphinx_doctests %}
 extras =
     docs

config/default/tox-testenv.j2

@@ -1,7 +1,7 @@
 
 [testenv]
 usedevelop = true
-{% if with_config_type == "pure-python" %}
+{% if config_type == "pure-python" %}
 package = wheel
 wheel_build_env = .pkg
 {% endif %}