Python 3.12 compatibility

[giampaolo]

Python 3.12 removed asyncore and asynchat modules. Copy them into our code base.

[codecov[bot]]

Codecov Report

Patch coverage: 62.50% and project coverage change: -0.27% :warning:

Comparison is base (a127d98) 69.96% compared to head (44c05cf) 69.69%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #605 +/- ## ========================================== - Coverage 69.96% 69.69% -0.27% ========================================== Files 9 9 Lines 3612 3617 +5 ========================================== - Hits 2527 2521 -6 - Misses 1085 1096 +11 ``` | [Files Changed](https://app.codecov.io/gh/giampaolo/pyftpdlib/pull/605?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Giampaolo+Rodola) | Coverage Δ | | |---|---|---| | [pyftpdlib/ioloop.py](https://app.codecov.io/gh/giampaolo/pyftpdlib/pull/605?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Giampaolo+Rodola#diff-cHlmdHBkbGliL2lvbG9vcC5weQ==) | `89.51% <60.00%> (-0.51%)` | :arrow_down: | | [pyftpdlib/handlers.py](https://app.codecov.io/gh/giampaolo/pyftpdlib/pull/605?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Giampaolo+Rodola#diff-cHlmdHBkbGliL2hhbmRsZXJzLnB5) | `70.38% <66.66%> (-0.18%)` | :arrow_down: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/giampaolo/pyftpdlib/pull/605/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Giampaolo+Rodola)

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[sbraz]

Hi @giampaolo, do you plan on releasing a new version including this commit for Python 3.12 support? I'd like to know if we need to release a snapshot of the current git master in Gentoo to support it or if we can wait until a future release.