I think it's nicer to make one last release with the remaining code
for python2 in place.

Based on Jörg Behrmann's comment:
https://github.com/systemd/python-systemd/pull/114#discussion_r945896280
> apt is available in the stretch, which is the oldest listed [1] release for it
> and stretch has already ended LTS support.

[1] https://packages.debian.org/bullseye/apt

Fedora has no python2 package, so drop that.
Similarly, with latest Debian and Opensuse, 'apt-get install
python3-systemd' works, but 'apt-get install python-systemd' doesn't.

(Tested with
podman run -it --rm debian apt-get install python3-systemd
mock -r opensuse-tumbleweed-x86_64 --enable-network -i python3-systemd)

It also seems we never built the package in EPEL, so let's drop
RHEL/CentOS from the list.

If people need to, they can install from sources where the distro
packages are unavailable (and they shouldn't be using python2 anyway
at this point.)

Also add 'apt-get update' into the instructions because w/o that
install does not work in a pristine container.

Fixes #78.

Bash doesn't do anything special for a command with '#', but at least
zsh treat it as a comment. It's better to quote such a commandline.

pip3 install says:
WARNING: Discarding git+https://github.com/systemd/python-systemd.git#egg=systemd. Requested systemd-python from git+https://github.com/systemd/python-systemd.git#egg=systemd has inconsistent name: filename has 'systemd', but metadata has
 'systemd-python'

Closes #97.

Journal examples

"libsytemd" should obviously be "libsystemd"