Re-organize directory structure (#4971)

See discussion in #2491

Co-authored-by: Ivan Levkivskyi <ilevkivskyi@dropbox.com>

stdlib/poplib.pyi

@@ -0,0 +1,73 @@
+import socket
+import ssl
+import sys
+from typing import Any, BinaryIO, Dict, List, Optional, Pattern, Text, Tuple, overload
+
+_LongResp = Tuple[bytes, List[bytes], int]
+
+class error_proto(Exception): ...
+
+POP3_PORT: int
+POP3_SSL_PORT: int
+CR: bytes
+LF: bytes
+CRLF: bytes
+
+class POP3:
+    if sys.version_info >= (3, 0):
+        encoding: Text
+
+    host: Text
+    port: int
+    sock: socket.socket
+    file: BinaryIO
+    welcome: bytes
+    def __init__(self, host: Text, port: int = ..., timeout: float = ...) -> None: ...
+    def getwelcome(self) -> bytes: ...
+    def set_debuglevel(self, level: int) -> None: ...
+    def user(self, user: Text) -> bytes: ...
+    def pass_(self, pswd: Text) -> bytes: ...
+    def stat(self) -> Tuple[int, int]: ...
+    def list(self, which: Optional[Any] = ...) -> _LongResp: ...
+    def retr(self, which: Any) -> _LongResp: ...
+    def dele(self, which: Any) -> bytes: ...
+    def noop(self) -> bytes: ...
+    def rset(self) -> bytes: ...
+    def quit(self) -> bytes: ...
+    def close(self) -> None: ...
+    def rpop(self, user: Text) -> bytes: ...
+    timestamp: Pattern[Text]
+
+    if sys.version_info < (3, 0):
+        def apop(self, user: Text, secret: Text) -> bytes: ...
+    else:
+        def apop(self, user: Text, password: Text) -> bytes: ...
+    def top(self, which: Any, howmuch: int) -> _LongResp: ...
+    @overload
+    def uidl(self) -> _LongResp: ...
+    @overload
+    def uidl(self, which: Any) -> bytes: ...
+    if sys.version_info >= (3, 5):
+        def utf8(self) -> bytes: ...
+    if sys.version_info >= (3, 4):
+        def capa(self) -> Dict[Text, List[Text]]: ...
+        def stls(self, context: Optional[ssl.SSLContext] = ...) -> bytes: ...
+
+class POP3_SSL(POP3):
+    if sys.version_info >= (3, 0):
+        def __init__(
+            self,
+            host: Text,
+            port: int = ...,
+            keyfile: Optional[Text] = ...,
+            certfile: Optional[Text] = ...,
+            timeout: float = ...,
+            context: Optional[ssl.SSLContext] = ...,
+        ) -> None: ...
+    else:
+        def __init__(
+            self, host: Text, port: int = ..., keyfile: Optional[Text] = ..., certfile: Optional[Text] = ..., timeout: float = ...
+        ) -> None: ...
+    if sys.version_info >= (3, 4):
+        # "context" is actually the last argument, but that breaks LSP and it doesn't really matter because all the arguments are ignored
+        def stls(self, context: Any = ..., keyfile: Any = ..., certfile: Any = ...) -> bytes: ...