gh-101100: Fix sphinx warnings in library/smtplib.rst

[LamentXU123]

smtplib.rst:81: WARNING: py:meth reference target not found: starttls [ref.meth]
smtplib.rst:113: WARNING: py:meth reference target not found: connect [ref.meth]
smtplib.rst:149: WARNING: py:attr reference target not found: smtp_code [ref.attr]
smtplib.rst:149: WARNING: py:attr reference target not found: smtp_error [ref.attr]
smtplib.rst:164: WARNING: py:attr reference target not found: recipients [ref.attr]
smtplib.rst:258: WARNING: py:attr reference target not found: helo_resp [ref.attr]
smtplib.rst:269: WARNING: py:attr reference target not found: ehlo_resp [ref.attr]
smtplib.rst:269: WARNING: py:attr reference target not found: does_esmtp [ref.attr]
smtplib.rst:269: WARNING: py:attr reference target not found: esmtp_features [ref.attr]
smtplib.rst:350: WARNING: py:attr reference target not found: esmtp_features [ref.attr]
smtplib.rst:419: WARNING: py:attr reference target not found: SSLContext.check_hostname [ref.attr]
smtplib.rst:431: WARNING: py:meth reference target not found: mail [ref.meth]
smtplib.rst:431: WARNING: py:meth reference target not found: rcpt [ref.meth]
smtplib.rst:431: WARNING: py:meth reference target not found: data [ref.meth]
smtplib.rst:470: WARNING: py:attr reference target not found: recipients [ref.attr]

• line 81. The method starttls is referring to SMTP.starttls (Because using this is inappropriate so we use SMTP_SSL)
• line 113. Same as line 81. I change this to ~SMTP.connect (Not sure. Maybe we can suppress this?)
• line 149. I document the attribute of the Exception to fix this:

.. attribute:: smtp_code

  The error code.

.. attribute:: smtp_error

  The error message.

• line 164. Same as line 149, I document the attribute:

.. attribute:: recipients

  The errors for each recipient are accessible through this
  attribute, which is a dictionary of exactly thesame sort as
  :meth:`SMTP.sendmail` returns.

• line 258,269,350. Those are undocumented attributes of object SMTP, also I write an index on the beginning of the document of this object:

.. attribute:: SMTP.helo_resp

   The response to the ``HELO`` command, see :meth:`helo`.


.. attribute:: SMTP.ehlo_resp
   
   The response to the ``EHLO`` command, see :meth:`ehlo`.


.. attribute:: SMTP.does_esmtp

   A boolean value indicating whether the server supports ESMTP, see
   :meth:`ehlo`.


.. attribute:: SMTP.esmtp_features

   A dictionary of the names of SMTP service extensions supported by the server,
   see :meth:`ehlo`.

Now the reference points to this attribute index.

• line 419. The SSLContext.check_hostname is actually referring to ssl.SSLContext.check_hostname, I change this to ssl.SSLContext.check_hostname
• line 431. They are undocumented low-level python-apis, I suppressed them all.
• line 470. I also create a index for this attibute: This is already documented in the fix of line 164.

• Issue: Fix all Sphinx reference warnings in the documentation #101100

---

📚 Documentation preview 📚: https://cpython-previews--139991.org.readthedocs.build/

[StanFromIreland]

LGTM

[miss-islington-app]

Thanks @LamentXU123 for the PR, and @hugovk for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-140084 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-140085 is a backport of this pull request to the 3.13 branch.