Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/asyncio/exceptions.pyi

@@ -0,0 +1,15 @@
+import sys
+from typing import Optional
+
+if sys.version_info >= (3, 8):
+    class CancelledError(BaseException): ...
+    class TimeoutError(Exception): ...
+    class InvalidStateError(Exception): ...
+    class SendfileNotAvailableError(RuntimeError): ...
+    class IncompleteReadError(EOFError):
+        expected: Optional[int]
+        partial: bytes
+        def __init__(self, partial: bytes, expected: Optional[int]) -> None: ...
+    class LimitOverrunError(Exception):
+        consumed: int
+        def __init__(self, message: str, consumed: int) -> None: ...