[Pkg-mailman-hackers] Bug#958976: mailman3: import21 is broken with python 3.7 and python 3.8

[Daniel Kahn Gillmor]

Package: mailman3
Version: 3.2.1-1
Severity: important
Control: tags -1 + patch fixed-upstream
Control: fixed -1 3.3.0
Control: forwarded -1 https://gitlab.com/mailman/mailman/-/issues/625

On a stock buster system, it is impossible to import a mailman2 mailing
list into mailman3, because the `import21` subcommand fails with the
a message like the following:

```
Traceback (most recent call last):
  File "/usr/bin/mailman", line 11, in <module>
    load_entry_point('mailman', 'console_scripts', 'mailman')()
  File "…/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "…/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "…/bin/mailman.py", line 68, in invoke
    return super().invoke(ctx)
  File "…/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "…/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "…/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "…/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "…/mailman/commands/cli_import.py", line 64, in import21
    pickle_file, encoding='utf-8', errors='ignore')
ModuleNotFoundError: No module named 'Mailman'
```

This is serious enough that it prevents anyone from the simplest path to
migrating from mailman2 to mailman3 on a buster system, so i think it
ought to be prioritized for a point release, especially as the
narrowly-targeted fix (attached here) is simple.

This was fixed upstream in 3.3.0.

See upstream fixes (where i extracted this particular fix from):
https://gitlab.com/mailman/mailman/-/commit/2a15437e911660ab87f960ac3a9eba131a2b7350
https://gitlab.com/mailman/mailman/-/commit/6bfe2d3bfa48ee391169eedaf2fbf024f5d7f423

Regards,

        --dkg

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