README: require python >= 3.7, clean up module descriptions

libpython was added in 2cc86f094a8c316f7feb0336df3827a3264b116d, it seems
because of python-systemd module that we built. But libpython by itself
is not enough for actual python programs, and now we also list python itself,
so let's drop libpython from the list.

meson requires >= 3.7. We have CI that runs on CentOS8 with Python 3.6, but
let's not provide official support for an EOL Python version. Individual
distributions can provide backports, but we don't need to mention that in
the user-facing docs. According to [1], 3.7 is on life support and 3.6 is EOL.

[1] https://devguide.python.org/versions/

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