bpo-32803: Handle DATA correctly for LMTP with multiple RCPT

[middagj]

Conform RFC 2033, the LMTP protocol gives for each successful recipient a reply.
The smtplib only reads one. This gives problems sending more than one message
with multiple recipients in a connection. See https://bugs.python.org/issue32803

To be backwards compatible, I added a new method multi_data to support a DATA command with multiple replies. I modeled it after data and wanted to keep the error handling in the sendmail method. Because we need to close the connection once we receive a 421, I chose to yield the results instead of returning a list. Similar to the multiple RCTP solution, I only raise an Exception if all recipients fail.

I am not sure about the naming of LMTPDataError. It breaks the structure of all errors beginning with LMTP, but it is really a specific LMTP error. An alternative could be SMTPMultiDataError.

I also fixed the assertion in test_421_from_data_cmd. I think it should compare rset_count and not rcpt_count. The rcpt_count is only modified if rcpt_response is used. The purpose of the test is to be sure the method closed instead of reset.

https://bugs.python.org/issue32803

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