Add a "doc" extra for documentation build dependencies

And use it in CI checks and in the Read the Docs configuration.

Author: EliahKagan

.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",