Allow authors to use different email addresses in different PEPs

The context here is that Yury Selivanov and I have a forthcoming PEP
and would prefer to use our current employer's email addresses for it,
but don't want to change our email addresses on earlier PEPs we've
written. (Obviously this affects more PEPs for Yury than for me :P)

There's no serious technical impediment to supporting this, so I have.

I've tested this locally.

Author: msullivan

pep_sphinx_extensions/pep_zero_generator/writer.py

@@ -331,36 +331,25 @@ def _classify_peps(peps: list[PEP]) -> tuple[list[PEP], ...]:
 
 
 def _verify_email_addresses(peps: list[PEP]) -> dict[str, str]:
-    authors_dict: dict[str, set[str]] = {}
+    authors_dict: dict[str, list[str]] = {}
     for pep in peps:
         for author in pep.authors:
             # If this is the first time we have come across an author, add them.
             if author.full_name not in authors_dict:
-                authors_dict[author.full_name] = set()
+                authors_dict[author.full_name] = []
 
             # If the new email is an empty string, move on.
             if not author.email:
                 continue
             # If the email has not been seen, add it to the list.
-            authors_dict[author.full_name].add(author.email)
-
-    valid_authors_dict: dict[str, str] = {}
-    too_many_emails: list[tuple[str, set[str]]] = []
-    for full_name, emails in authors_dict.items():
-        if len(emails) > 1:
-            too_many_emails.append((full_name, emails))
-        else:
-            valid_authors_dict[full_name] = next(iter(emails), "")
-    if too_many_emails:
-        err_output = []
-        for author, emails in too_many_emails:
-            err_output.append(" " * 4 + f"{author}: {emails}")
-        raise ValueError(
-            "some authors have more than one email address listed:\n"
-            + "\n".join(err_output)
-        )
-
-    return valid_authors_dict
+            emails = authors_dict[author.full_name]
+            if author.email not in emails:
+                emails.append(author.email)
+
+    # Combine multiple email addresses with commas. Since peps is
+    # sorted by PEP number, this should produce a deterministic
+    # output.
+    return {name: ', '.join(emails) for name, emails in authors_dict.items()}
 
 
 def _sort_authors(authors_dict: dict[str, str]) -> list[str]: