imaplib: Fix annotation for search. (#1762)

* imaplib: Fix annotation for search.

https://docs.python.org/3/library/imaplib.html#imaplib.IMAP4.search
shows that IMAP4.search takes a number of str args, not a number of
List[str] args (I can see how one would be confused, though, because
of the *criteria).

* imaplib: Fix annotation for store.

While it's called "flag_list",
https://docs.python.org/3/library/imaplib.html#imaplib.IMAP4.store
clearly shows it's expecting a string with a list of flags in it, not
a Python list.

stdlib/2and3/imaplib.pyi

@@ -69,7 +69,7 @@ class IMAP4:
     def partial(self, message_num: str, message_part: str, start: str, length: str) -> CommandResults: ...
     def proxyauth(self, user: str) -> CommandResults: ...
     def rename(self, oldmailbox: str, newmailbox: str) -> CommandResults: ...
-    def search(self, charset: str, *criteria: List[str]) -> CommandResults: ...
+    def search(self, charset: str, *criteria: str) -> CommandResults: ...
     def select(self, mailbox: str = ..., readonly: bool = ...) -> CommandResults: ...
     def setacl(self, mailbox: str, who: str, what: str) -> CommandResults: ...
     def setannotation(self, *args: List[str]) -> CommandResults: ...
@@ -78,7 +78,7 @@ class IMAP4:
     if sys.version_info >= (3,):
         def starttls(self, ssl_context: Optional[Any] = ...) -> CommandResults: ...
     def status(self, mailbox: str, names: str) -> CommandResults: ...
-    def store(self, message_set: str, command: str, flags: List[str]) -> CommandResults: ...
+    def store(self, message_set: str, command: str, flags: str) -> CommandResults: ...
     def subscribe(self, mailbox: str) -> CommandResults: ...
     def thread(self, threading_algorithm: str, charset: str, *search_criteria: List[str]) -> CommandResults: ...
     def uid(self, command: str, *args: List[str]) -> CommandResults: ...