Suggest removing atty dependency

[EddieWhi]

atty is unmaintained and has outstanding sec advisory RUSTSEC-2021-0145.

softprops/atty#50

As removal of the dependency is simple, this seems like the obvious solution.

Copied from the atty source here:
https://github.com/softprops/atty/blob/6633c0e1446aa19e6cd00e00e39770da43081bda/src/lib.rs#L40

This should be the exact equivalent but using libc directly.

Hope you don't mind the surprise PR @softprops... this just came up on cargo deny on an internal repo

[softprops]

@EddieWhi no worries. I carry the blame on that one but I think there's actually a better way to address this.

atty is technically provides no additional value over use std::io::IsTerminal introduced in rust 1.7.0.

What are your thoughts on using std::io::stdout().is_terminal() to remove the atty dependency? I think this should be roughly equiv and let's the std lib to do the portability work for us.

[EddieWhi]

TIL about IsTerminal, thank you 👍

Turns out it is not just roughly equivalent, it's identical: https://docs.rs/is-terminal/latest/src/is_terminal/lib.rs.html#90

Arguably, there was something pleasant about the symmetry of:

    unsafe { isatty(STDOUT_FILENO...
...
    let r = unsafe { ioctl(STDOUT_FILENO...

but really can't turn down a std lib function :)

Updated and suggest adding min supported rust-version to Cargo.toml, WDYT?

[softprops]

thanks @EddieWhi this should be up on crates.io now