Disable cygwin for now

[jaraco]

Workaround for #3921.

CC @lazka : Would you be willing to look into what it will take to make a rust compiler available in the test_cygwin?

[abravalheri]

@jaraco, how about we use something like the following in setup.cfg:

[options.extras_require]
testing =
   # ...
   pytest-ruff; sys_platform != "cygwin"

(i.e. instead of disabling the whole cygwin test, we rely on pytest-enabler not finding it installed and skipping the pytest plugin, but still running the majority of tests).

Would that be a valid workaround?

[jaraco]

Good idea. It's still not a precise match, because in theory it should work, but definitely a superior approach.

[lazka]

(Rust doesn't support cygwin, so that's unlikely to work)

[DWesl]

what it will take to make a rust compiler available in the test_cygwin?

Add Cygwin as a target to the rust codebase, create a cross-compiler on an existing target, then use that to create a native compiler
rust-lang/rust#79854 (comment)

Alternately, try to get mrustc working on Cygwin
https://github.com/thepowersgang/mrustc