Use new key for setting readthedocs python version

.readthedocs.yml

@@ -1,7 +1,6 @@
 version: 2
 
 python:
-   version: 3.7
    install:
       - requirements: doc/en/requirements.txt
       - method: pip
@@ -9,6 +8,8 @@ python:
 
 build:
   os: ubuntu-20.04
+  tools:
+    python: "3.9"
   apt_packages:
     - inkscape
 

doc/en/conf.py

@@ -43,7 +43,8 @@ todo_include_todos = 1
 latex_engine = "lualatex"
 
 latex_elements = {
-    'preamble': dedent("""
+    "preamble": dedent(
+        r"""
         \directlua{
             luaotfload.add_fallback("fallbacks", {
                 "Noto Serif CJK SC:style=Regular;",
@@ -52,7 +53,8 @@ latex_elements = {
         }
 
         \setmainfont{FreeSerif}[RawFeature={fallback=fallbacks}]
-    """)
+        """
+    )
 }
 
 # -- General configuration -----------------------------------------------------