Replace unmaintained fs2 with fs4

[messense]

No description provided.

[messense]

This change is part of the following stack:

• Replace unmaintained fs2 with fs4 #2317 ◀
  • Fix some test-crates build #2318

_{Change managed by git-spice.}

[messense]

@konstin any idea why cffi venv related tests always fail on Windows?

[konstin]

not sure, sorry, i can try to debug on windows later if the errors persist.

[messense]

Found the culprit:

maturin/src/target.rs

Lines 633 to 644 in ea2e54e

	let bin_dir = venv.join("Scripts");
	if bin_dir.join("python.exe").exists() {
	return bin_dir;
	}
	// Python installed via msys2 on Windows might produce a POSIX-like venv
	// See https://github.com/PyO3/maturin/issues/1108
	let bin_dir = venv.join("bin");
	if bin_dir.join("python.exe").exists() {
	return bin_dir;
	}
	// for conda environment
	venv.to_path_buf()

get_venv_bin_dir can only works correctly with a existing venv on Windows.