Ensure the "classes" extra is installed in tests.

Author: domdfcoding

.readthedocs.yml

@@ -15,3 +15,5 @@ python:
    - requirements: doc-source/requirements.txt
    - method: pip
      path: .
+     extra_requirements:
+      - classes

repo_helper.yml

@@ -14,6 +14,7 @@ use_whey: true
 python_deploy_version: 3.6
 docs_fail_on_warning: true
 mypy_version: "0.812"
+tox_testenv_extras: classes
 
 conda_channels:
  - conda-forge

tox.ini

@@ -33,6 +33,7 @@ setenv = SHOW_TODOS = 1
 basepython = python3.8
 pip_version = pip>=21
 changedir = {toxinidir}/doc-source
+extras = classes
 deps = -r{toxinidir}/doc-source/requirements.txt
 commands = sphinx-build -M {env:SPHINX_BUILDER:html} . ./build {posargs}
 
@@ -78,6 +79,7 @@ commands = python3 -m flake8_rst_docstrings_sphinx flake8_encodings tests --allo
 basepython = python3.6
 ignore_errors = True
 changedir = {toxinidir}
+extras = classes
 deps =
     mypy==0.812
     -r{toxinidir}/tests/requirements.txt
@@ -90,6 +92,7 @@ skip_install = True
 ignore_errors = True
 changedir = {toxinidir}
 deps = pyupgrade-directories
+extras = classes
 commands = pyup_dirs flake8_encodings tests --py36-plus --recursive
 
 [testenv:coverage]