Update Python and Django supported versions

[pfouque]

Update CI to current django versions

• Add Python 3.13, 3.14 support
• Add Django 5.1, 5.2, 6.0 support
• Remove support for deprecated Django versions

Fix reply message parsing in Django 6 by avoiding legacy round‑trip

Root cause
Django 6 switched EmailMessage.message() to return Python’s modern email.message.EmailMessage. We were serializing that object to a string and re‑parsing it with email.message_from_string(), which produces a legacy compat32 Message. That round‑trip dropped/failed to expose the In-Reply-To header, so _process_message never set in_reply_to, causing test_message_reply to fail.

Fix
Skip the serialize/parse round‑trip and pass the EmailMessage object directly into record_outgoing_message. This preserves the In-Reply-To header and makes reply handling work consistently on Django 6+.

NB: I dunno why this round‑trip was implemented in a first place, but all tests are passing now...

[coddingtonbear]

I might be misunderstanding what you wrote in your PR description, but won't this only work for Django 6+, while I believe you've left the repo set to continue supporting 5?

[pfouque]

Hi @coddingtonbear,
the main change in Django 6.0 is that message.message() now returns a <class 'email.message.EmailMessage'> instead of <class 'django.core.mail.message.SafeMIMEText'>.
But the new class, when serialized-deserialized as string, does not preserve custom headers like In-Reply-To.

I started with a Django 6 only fix, but it appeared that the serialization-deserialization wasn't useful (according to the test suite)
But I can definitely add a statement to apply it only for Django6+