rust: remove `atty` dependency
It is fully replaced with the now stable `std::io::IsTerminal` trait.
This was the last dependency flagged as a warning by `cargo audit`,
aside from `cpython` which we know about all too well: the plan is to
transition to PyO3 soon-ish.
#require emacs
$ emacs -q -no-site-file -batch -l $TESTDIR/../contrib/hg-test-mode.el \
> -f ert-run-tests-batch-and-exit
Running 1 tests (*) (glob)
passed 1/1 hg-test-mode--compilation-mode-support* (glob)
Ran 1 tests, 1 results as expected* (glob)