gh-101100: Fix Sphinx warnings in library/email.compat32-message.rst

[LamentXU123]

before:

C:\Users\admin\Downloads\cpython-main\Doc\library\email.compat32-message.rst:226: WARNING: py:class reference target not found: email.emailmessage.EmailMessage [ref.class]
C:\Users\admin\Downloads\cpython-main\Doc\library\email.compat32-message.rst:238: WARNING: py:class reference target not found: email.emailmessage.EmailMessage [ref.class]
C:\Users\admin\Downloads\cpython-main\Doc\library\email.compat32-message.rst:267: WARNING: py:class reference target not found: email.emailmessage.EmailMessage [ref.class]
C:\Users\admin\Downloads\cpython-main\Doc\library\email.compat32-message.rst:267: WARNING: py:meth reference target not found: email.emailmessage.EmailMessage.set_content [ref.meth]
C:\Users\admin\Downloads\cpython-main\Doc\library\email.compat32-message.rst:278: WARNING: py:class reference target not found: email.emailmessage.EmailMessage [ref.class]
C:\Users\admin\Downloads\cpython-main\Doc\library\email.compat32-message.rst:488: WARNING: py:class reference target not found: email.emailmessage.EmailMessage [ref.class]
C:\Users\admin\Downloads\cpython-main\Doc\library\email.compat32-message.rst:526: WARNING: py:class reference target not found: email.emailmessage.EmailMessage [ref.class]
C:\Users\admin\Downloads\cpython-main\Doc\library\email.compat32-message.rst:581: WARNING: py:class reference target not found: email.emailmessage.EmailMessage [ref.class]

The warnings are raised in a same reason. An example:

      This is a legacy method.  On the
      :class:`~email.emailmessage.EmailMessage` class its functionality is
      replaced by :meth:`~email.message.EmailMessage.get_content` and
      :meth:`~email.message.EmailMessage.iter_parts`.

The referrence to EmailMessage is invalid, because the moudle is actually email.message.EmailMessage not email.emailmessage.EmailMessage . This PR fix them all.

After:

and the example:

In this example, you can check the set_content method, which is a method of email.message.EmailMessage not email.emailmessage.EmailMessage

The preview doc of this moudle: https://cpython-previews--136323.org.readthedocs.build/en/136323/library/email.compat32-message.html?readthedocs-diff=true&readthedocs-diff-chunk=1

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

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📚 Documentation preview 📚: https://cpython-previews--136323.org.readthedocs.build/