I have reviewed RFC 3339 and it looks like the following produces a fully compliant timestamp:
>>> print(datetime(2000,1,1, tzinfo=timezone.utc).isoformat('T'))
2000-01-01T00:00:00+00:00
I see the following remaining issues:
1. It is often desired to get RFC 3339 timestamp in local timezone instead of UTC. This will be addressed in issue 9527.
2. If UTC timestamp is produced by a computer in non-UTC timezone, the offset should be specified as '-00:00'. If this is important, an application can replace '+' with '-', but most likely specifying the correct local offset is a better option.
3. RFC 3339 requires support for leap seconds. This limitation cannot be solved by adding a method to datetime.
Most importantly, given that there are several RFCs describing different date formats, a datetime.rfcformat() method will be ambiguous. (GNU date has --rfc-2822 and --rfc-3339 options and the later allows output of three different precisions.)
I am going to reject this RFE. I don't think adding datetime.rfcformat() method will solve any real deficiency and whatever limitations datetime has with respect to producing RFC compliant timestamps should be addressed in future specific proposals.