gh-159631: Reject NUL in memoryview credentials in IMAP4.login_plain()

[fedonman]

login_plain() now converts the user name and password to bytes before checking them for NUL, so the check also applies to a memoryview or another bytes-like object.

$ ./python -m test test_imaplib -u network
...
1 test OK.

Total duration: 56.0 sec
Total tests: run=263
Total test files: run=1/1
Result: SUCCESS

• Issue: IMAP4.login_plain() does not reject NUL in a memoryview credential #159631