bpo-43501: _header_value_parser: handle DOT in DisplayName

[elenril]

https://bugs.python.org/issue43501

[the-knights-who-say-ni]

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[github-actions]

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

[elenril]

Ping.

Still waiting for review.

[akulakov]

The question is, should this be parsed or should it raise a more informative error, e.g. something like "name should be quoted"?

Can you find the relevant RFC rule?

I tested it and if the name is quoted, it's parsed successfully:

msg = EmailMessage()
msg["to"] = '"name."<a@b.com>'
print (msg.as_string())

to: "name."<a@b.com>

[elenril]

Honestly I see little point in spending effort on finding whether this might or might not be compatible with some obscure obsolete syntax or not. According to the comments in the source this already adds an
InvalidHeaderDefect, and I've only seen this happen in broken spam emails, so I only care about Python not leaking random exceptions. How exactly such emails are handled does not matter to me. If you, or anyone else,
has an opinion on how precisely to handle this, then suggestions are welcome. I personally have no opinion either way.

[elenril]

Ping.
Would appreciate some progress on this.

[elenril]

Ping.
Rebased against current upstream.