Use Ubuntu 20.04, lualatex for readthedocs builds (#9242)

.readthedocs.yml

@@ -1,13 +1,15 @@
 version: 2
 
 python:
-   version: 3.7
    install:
       - requirements: doc/en/requirements.txt
       - method: pip
         path: .
 
 build:
+  os: ubuntu-20.04
+  tools:
+    python: "3.9"
   apt_packages:
     - inkscape
 

changelog/9242.doc.rst

@@ -0,0 +1 @@
+Upgrade readthedocs configuration to use a [newer Ubuntu version](https://blog.readthedocs.com/new-build-specification/) with better unicode support for PDF docs.

doc/en/conf.py

@@ -19,6 +19,7 @@ import ast
 import os
 import shutil
 import sys
+from textwrap import dedent
 from typing import List
 from typing import TYPE_CHECKING
 
@@ -39,9 +40,22 @@ autodoc_member_order = "bysource"
 autodoc_typehints = "description"
 todo_include_todos = 1
 
-# Use a different latex engine due to possible Unicode characters in the documentation:
-# https://docs.readthedocs.io/en/stable/guides/pdf-non-ascii-languages.html
-latex_engine = "xelatex"
+latex_engine = "lualatex"
+
+latex_elements = {
+    "preamble": dedent(
+        r"""
+        \directlua{
+            luaotfload.add_fallback("fallbacks", {
+                "Noto Serif CJK SC:style=Regular;",
+                "Symbola:Style=Regular;"
+            })
+        }
+
+        \setmainfont{FreeSerif}[RawFeature={fallback=fallbacks}]
+        """
+    )
+}
 
 # -- General configuration -----------------------------------------------------