MIMEBase parameter for EmailMessage.attach() (#577)

* MIMEBase parameter for EmailMessage.attach()

* Added test_mail.yml

Co-authored-by: Edwin Grubbs <edwin.grubbs@motiva.com>

tests/typecheck/test_mail.yml

@@ -0,0 +1,19 @@
+-   case: check_email_message_attach
+    main: |
+        from email.mime.text import MIMEText
+        from email.mime.image import MIMEImage
+        from django.core.mail.message import EmailMessage
+
+        message = EmailMessage()
+        reveal_type(message)  # N: Revealed type is 'django.core.mail.message.EmailMessage'
+
+        message.attach("myfilename", "mycontent", "text/plain")
+
+        mime_text = MIMEText("mytext")
+        reveal_type(mime_text)  # N: Revealed type is 'email.mime.text.MIMEText'
+        message.attach(mime_text)
+
+        mime_image = MIMEImage("mydata", "image/png")
+        reveal_type(mime_image)  # N: Revealed type is 'email.mime.image.MIMEImage'
+        message.attach(mime_image)
+        reveal_type(message.attachments)  # N: Revealed type is 'builtins.list[Any]'