Installation instructions are outdated

[oskar456]

In the installation instructions, there is a lot of obsolete information:

• The minimum supported version of Python is now 3.6, not 2.7. Though 3.6 is now EoL as well.
• Debian Jessie reached EoL in 2020.
• The command easy_install does not exist on macOS anymore.
• There is no Gentoo ebuild for ripe-atlas-tools anymore.
• I haven't checked OpenBSD and FreeBSD.

Perhaps the document should be simplified to require less maintenance over time.