I agree that SSLError should have used a different attribute, but it's too late for that - changing it would break any code currently relying on SSL errnos (in particular asynchronous code using the SSL_ERROR_WANT_{READ,WRITE} error codes for normal operation).

errno_scope is better than checking for isinstance(SSLError) because it lets code that only wants posix errno to say so, instead of having to know about and blacklist every OSError subclass that abuses the errno attribute (is SSLError the only one?)