imaplib.pyi: Add missing imaplib.IMAP4.enable() (#3314)

Closes: #3313

Author: utkarsh2102

stdlib/2and3/imaplib.pyi

@@ -52,6 +52,8 @@ class IMAP4:
     def create(self, mailbox: str) -> CommandResults: ...
     def delete(self, mailbox: str) -> CommandResults: ...
     def deleteacl(self, mailbox: str, who: str) -> CommandResults: ...
+    if sys.version_info >= (3, 5):
+        def enable(self, capability: str) -> CommandResults: ...
     def expunge(self) -> CommandResults: ...
     def fetch(self, message_set: str, message_parts: str) -> CommandResults: ...
     def getacl(self, mailbox: str) -> CommandResults: ...