imaplib fails when server sends extra blank line after literal value

[jrlevine]

BPO	44408
Nosy	@iritkatriel, @jrlevine
PRs	gh-88574: Handle extra blank response line in imaplib #26701

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-06-13.05:34:08.636>
labels = ['type-bug', 'library', '3.9', '3.10']
title = 'imaplib fails when server sends extra blank line after literal value'
updated_at = <Date 2022-01-19.19:33:36.808>
user = 'https://github.com/jrlevine'

bugs.python.org fields:

activity = <Date 2022-01-19.19:33:36.808>
actor = 'jrlevine'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-06-13.05:34:08.636>
creator = 'jrlevine'
dependencies = []
files = []
hgrepos = []
issue_num = 44408
keywords = ['patch']
message_count = 3.0
messages = ['395729', '410655', '410976']
nosy_count = 2.0
nosy_names = ['iritkatriel', 'jrlevine']
pr_nums = ['26701']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44408'
versions = ['Python 3.9', 'Python 3.10']

[jrlevine]

Some IMAP servers return an extra blank line after a counted literal value, which makes imaplib crash.
MacOS mail and T'bird handle the blank line OK so it seems to be a somewhat common bug.

[iritkatriel]

How does it fail? Can you provide a reproducer and add a unit test to the PR?