RUSTSEC-2020-0168: mach is unmaintained

[github-actions]

mach is unmaintained

Details
Status	unmaintained
Package	mach
Version	0.1.2
URL	fitzgen/mach#63
Date	2020-07-14

Last release was almost 4 years ago.

Maintainer(s) seem to be completely unreachable.

Possible Alternative(s)

These may or may not be suitable alternatives and have not been vetted in any way;

• mach2 - direct fork

See advisory page for additional details.

[stphnt]

This is an indirect dependency so it can only be will be fixed after serialport-rs makes a release including a fix for serialport/serialport-rs/issues/112