DOC: Require a more recent version of sphinx (#1923)

* update sphinx to use at least version 7
* downgrade the min-sphinx version but upgrade `sphinx-book-theme`
* upgrade to python 3.11

Author: keewis

.readthedocs.yaml

@@ -2,7 +2,7 @@ version: 2
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.9"
+    python: "3.11"
 sphinx:
   configuration: docs/conf.py
   fail_on_warning: false

requirements_docs.txt

@@ -1,4 +1,4 @@
-sphinx>4
+sphinx>=6
 ipython<=8.12
 matplotlib
 mip>=1.13
@@ -16,7 +16,7 @@ dask[complete]
 setuptools>=41.2
 Serialize
 pygments>=2.4
-sphinx-book-theme==0.3.3
+sphinx-book-theme>=1.1.0
 sphinx_copybutton
 sphinx_design
 typing_extensions