refactor/bug: handle case where netrc returns None.

pyrefly flagged this. Python 3.14 (and 2.7) says authenticators()
returns None if there is no matching host.  Also the original
exceptions IOError and TypeError appear to not be valid in this
context.

Rewrite to scope FileNotFoundError if .netrc is missing to just the
one method call. Handle rest of flow with if/else not exception
jump.

Author: rouilj

roundup/scripts/roundup_mailgw.py

@@ -215,15 +215,20 @@ def main(argv):
 
     # the source will be a network server, so obtain the credentials to
     # use in connecting to the server
+    authenticator = None
     try:
         # attempt to obtain credentials from a ~/.netrc file
+        # returns None if host not found
         authenticator = netrc.netrc().authenticators(specification)
+    except FileNotFoundError:
+        # FileNotFoundError if no ~/.netrc file
+        pass
+
+    if authenticator:
         username = authenticator[0]
         password = authenticator[2]
         server = specification
-        # IOError if no ~/.netrc file, TypeError if the hostname
-        # not found in the ~/.netrc file:
-    except (IOError, TypeError):
+    else:
         match = re.match(r'((?P<user>[^:]+)(:(?P<pass>.+))?@)?(?P<server>.+)',
                          specification)
         if match: