python: Use a real parser for email addresses.

Now that we can assume Python 3.6+, we can use the
email.headerregistry module to replace hacky manual email address
parsing.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/forms.py

@@ -1,5 +1,6 @@
 import logging
 import re
+from email.headerregistry import Address
 from typing import Any, Dict, List, Optional, Tuple
 
 import DNS
@@ -39,7 +40,6 @@ from zerver.models import (
     EmailContainsPlusError,
     Realm,
     UserProfile,
-    email_to_domain,
     get_realm,
     get_user_by_delivery_email,
     is_cross_realm_bot_email,
@@ -68,11 +68,11 @@ PASSWORD_TOO_WEAK_ERROR = gettext_lazy("The password is too weak.")
 
 def email_is_not_mit_mailing_list(email: str) -> None:
     """Prevent MIT mailing lists from signing up for Zulip"""
-    if "@mit.edu" in email:
-        username = email.rsplit("@", 1)[0]
+    address = Address(addr_spec=email)
+    if address.domain == "mit.edu":
         # Check whether the user exists and can get mail.
         try:
-            DNS.dnslookup(f"{username}.pobox.ns.athena.mit.edu", DNS.Type.TXT)
+            DNS.dnslookup(f"{address.username}.pobox.ns.athena.mit.edu", DNS.Type.TXT)
         except DNS.Base.ServerError as e:
             if e.rcode == DNS.Status.NXDOMAIN:
                 # This error is mark_safe only because 1. it needs to render HTML
@@ -240,7 +240,7 @@ def email_not_system_bot(email: str) -> None:
 
 
 def email_is_not_disposable(email: str) -> None:
-    if is_disposable_domain(email_to_domain(email)):
+    if is_disposable_domain(Address(addr_spec=email).domain):
         raise ValidationError(_("Please use your real email address."))