I played with the server and group that is apparently used in the test:

>>> server = NNTP_SSL("nntp.aioe.org")
>>> [_, _, first, last, _] = server.group("fr.comp.lang.python")
>>> first
2900
>>> last
2915
>>> server.body(last)[1].lines[-1]
b''
>>> server.body(first)[1].lines[-1]
b'Merci'
>>> server.body(last - 1)[1].lines[-1]
b'Kevin'

I tried all articles from 2900–2915, and none of them end with a dot on its own line, so I don’t actually know what caused the failure.

However looking again at the _getlongresp() implementation, I cannot imagine how the test failure can occur except for a genuine body that ends with a dot on its own line (encoded as b".." and then converted to b"."). Here is a quick patch to alter the test cases.