readthodocs: fully specify a build environment

readthodocs: fully specify a build environment

This is now expected by rtd so I've expanded using their example as
22.04 is one of our supported platforms. I tried to work out if there
was an easy way to re-generate a requirements.txt from our
pythondeps.toml but in the end went for the easier solution.

Cc:  <qemu-stable@nongnu.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231221174200.2693694-1-alex.bennee@linaro.org>
(cherry picked from commit b16a45bc5e0e329a16af8a2e020a6e7044f9afa2)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>