bpo-30514: test_pop3lib asyncore with asyncio

[grzgrzgrz3]

Pop3 server stub was base on asyncore/asynchat which is deprecated, new stub is using asyncio instead.

test_too_long_lines make sure we read remaining overlong response

cmd_user answer only with +ERR message on invalid user

#74699

[mention-bot]

@grzgrzgrz3, thanks for your PR! By analyzing the history of the files in this pull request, we identified @tiran, @serhiy-storchaka and @giampaolo to be potential reviewers.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@9c972b5). Click here to learn what that means. The diff coverage is 97.67%.

@@            Coverage Diff            @@
##             master    #1864   +/-   ##
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  Coverage          ?   82.68%           
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  Files             ?     1432           
  Lines             ?   353405           
  Branches          ?        0           
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  Hits              ?   292229           
  Misses            ?    61176           
  Partials          ?        0

Impacted Files	Coverage Δ
Lib/test/test_poplib.py	98.13% <97.67%> (ø)

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[github-actions[bot]]

This PR is stale because it has been open for 30 days with no activity.

[JelleZijlstra]

I'll work on getting this up to date so it unblocks removing asyncore. Note to self: local branch name is bpo30514-grzegorz-grzywacz-poplib.

[AA-Turner]

Triggering CLA.

[github-actions[bot]]

This PR is stale because it has been open for 30 days with no activity.

[github-actions[bot]]

This PR is stale because it has been open for 30 days with no activity.

[github-actions[bot]]

This PR is stale because it has been open for 30 days with no activity.