Add a "doc" extra for documentation build dependencies

And use it in:

- GitHub Actions CI checks
- Read the Docs configuration
- tox (for the "html" environment)

(The tox "html" environment was not previously overriding "extras"
to empty it out of dependencies needed only for linting or testing,
so this happens to make `tox -e html` faster.)

.github/workflows/pythonpackage.yml

@@ -100,5 +100,5 @@ jobs:
 
     - name: Documentation
       run: |
-        pip install -r doc/requirements.txt
+        pip install ".[doc]"
         make -C doc html

.readthedocs.yaml

@@ -32,4 +32,5 @@ python:
   install:
   - method: pip
     path: .
-  - requirements: doc/requirements.txt
+    extra_requirements:
+    - doc

setup.py

@@ -17,6 +17,7 @@ def _read_content(path: str) -> str:
 version = _read_content("VERSION").strip()
 requirements = _read_content("requirements.txt").splitlines()
 test_requirements = _read_content("test-requirements.txt").splitlines()
+doc_requirements = _read_content("doc/requirements.txt").splitlines()
 long_description = _read_content("README.md")
 
 
@@ -75,7 +76,10 @@ def _stamp_version(filename: str) -> None:
     package_dir={"git": "git"},
     python_requires=">=3.7",
     install_requires=requirements,
-    extras_require={"test": test_requirements},
+    extras_require={
+        "test": test_requirements,
+        "doc": doc_requirements,
+    },
     zip_safe=False,
     long_description=long_description,
     long_description_content_type="text/markdown",

tox.ini

@@ -23,7 +23,7 @@ ignore_outcome = true
 [testenv:html]
 description = Build HTML documentation
 base_python = py{39,310,311,312,38,37}
-deps = -r doc/requirements.txt
+extras = doc
 allowlist_externals = make
 commands =
     make BUILDDIR={env_tmp_dir}/doc/build -C doc clean