readthedocs: specify Python version and OS

Currently, readthedocs uses Python 3.7.9 by default. labgrid dropped
Python 3.7 support in labgrid-project#1129. Since this PR was merged, the readthedocs
builds fail [1]. This is especially annoying since labgrid-project#1079 was merged
afterwards, changing labgrid's setup and its documentation
significantly, meaning latest master and the docs hosted on readthedocs
differ in incompatible ways. labgrid users already stumbled upon this.

It is not really clear why this Python version is used, though: The docs
don't specify a default for build.tools.python [2], the legacy option
python.version [3] uses "3" as default, which seems to correlate with the
value documented in [2] meaning "last stable CPython version". But 3.7.9
is not the "last stable CPython version".

To fix this, configure all required options (including build.os) to
allow setting the Python version. Set the Python version to the latest
stable version 3.11 and the OS version to the latest available value
ubuntu-22.04.

[1] https://readthedocs.org/projects/labgrid/builds/
[2] https://docs.readthedocs.io/en/stable/config-file/v2.html#build-tools-python
[3] https://docs.readthedocs.io/en/stable/config-file/v2.html#python-version-legacy

Signed-off-by: Bastian Krause <bst@pengutronix.de>

.readthedocs.yaml

@@ -6,3 +6,8 @@ python:
       path: .
       extra_requirements:
         - doc
+
+build:
+  os: "ubuntu-22.04"
+  tools:
+    python: "3.11"