Build user's guide: bump to Python 3.10

(cherry picked from commit 3b6e0f21)

.github/workflows/users-guide.yml

@@ -46,7 +46,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.7]
+        python-version: ['3.10']
 
     steps:
     - uses: actions/checkout@v3

doc/Makefile

@@ -23,6 +23,7 @@ build-and-check-requirements: requirements.txt check-requirements
 # See https://modelpredict.com/wht-requirements-txt-is-not-enough
 requirements.txt: requirements.in
 	. ../.python-sphinx-virtualenv/bin/activate \
+	  && pip install --upgrade pip \
 	  && pip install pip-tools \
 	  && pip-compile requirements.in
 

doc/requirements.txt

 #
-# This file is autogenerated by pip-compile with python 3.9
-# To update, run:
+# This file is autogenerated by pip-compile with Python 3.10
+# by the following command:
 #
 #    pip-compile requirements.in
 #
@@ -21,8 +21,6 @@ idna==2.10
     # via requests
 imagesize==1.2.0
     # via sphinx
-importlib-metadata==4.11.4
-    # via sphinx
 jinja2==2.11.3
     # via sphinx
 jsonpointer==2.1
@@ -72,5 +70,3 @@ sphinxnotes-strike==1.1
     # via -r requirements.in
 urllib3==1.26.7
     # via requests
-zipp==3.8.0
-    # via importlib-metadata