gh-119511: Fix a potential denial of service in imaplib (#119514)

The IMAP4 client could consume an arbitrary amount of memory when trying
to connect to a malicious server, because it read a "literal" data with a
single read(size) call, and BufferedReader.read() allocates the bytes
object of the specified size before reading. Now the IMAP4 client reads data
by chunks, therefore the amount of used memory is limited by the
amount of the data actually been sent by the server.

Co-authored-by: Gregory P. Smith <greg@krypto.org>