Not sure if this is the cause here, but maturing defaults to "cygwin_86_64" because of me since https://github.com/PyO3/maturin/pull/2819 I suggested on this list some months ago to change sysconfig to match it like: https://gist.github.com/lazka/c8146c08c0ebe5beb43b6ffb6c2ae51b so pip stays happy across cygwin DLL updates