Install manpage and bump python to 3.10+

[branchvincent]

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This installs the manpage at {venv}/share/man/man1, which is recognized by installers like pipx and homebrew (with uv to hopefully follow soon).

Also, I bumped python-requires to 3.8+, which matches the earliest Ubuntu LTS (20.04) following https://github.com/rfinnie/2ping/commit/80e9766d72a05f614e8fc581d2f6ee5c976acf59. Note that the project was no longer installable with 3.6 anyway since setuptools introduced experimental support for pyproject.toml in v61 (3.7+) and later stabilized in v68.1 (3.8+)

[branchvincent]

Actually, I see now that 22.04 is the earliest LTS which is 3.10 so let's do that? Also allows updating to PEP 639 license metadata