Using the new ensuregroup command, the desired versions of meson and
sphinx can be placed in pythondeps.toml rather than configure.

The meson.install entry in pythondeps.toml matches the version that is
found in python/wheels.  This ensures that mkvenv.py uses the bundled
wheel even if PyPI is enabled; thus not introducing warnings or errors
from versions that are more recent than the one used in CI.

The sphinx entries match what is shipped in Fedora 38.  It's the
last release that has support for older versions of Python (sphinx 6.0
requires Python 3.8) and especially docutils (of which sphinx 6.0 requires
version 0.18).  This is important because Ubuntu 20.04 has docutils 0.14
and Debian 11 has docutils 0.16.

"mkvenv.py ensure" is only used to bootstrap tomli.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>