Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/asyncio/constants.pyi

@@ -0,0 +1,14 @@
+import enum
+import sys
+
+LOG_THRESHOLD_FOR_CONNLOST_WRITES: int
+ACCEPT_RETRY_DELAY: int
+DEBUG_STACK_DEPTH: int
+if sys.version_info >= (3, 7):
+    SSL_HANDSHAKE_TIMEOUT: float
+    SENDFILE_FALLBACK_READBUFFER_SIZE: int
+
+class _SendfileMode(enum.Enum):
+    UNSUPPORTED: int = ...
+    TRY_NATIVE: int = ...
+    FALLBACK: int = ...