Add missing attributes: asyncio.subprocess.Process

Add typehints for the following `asyncio.subprocess.Process` attributes:

- `stdin`: `Optional[asyncio.streams.StreamWriter]`
- `stdout`: `Optional[asyncio.streams.StreamReader]`
- `stderr`: `Optional[asyncio.streams.StreamReader]`
- `pid`: `int`

Author: sdunster

stdlib/3.4/asyncio/subprocess.pyi

@@ -21,6 +21,10 @@ class SubprocessStreamProtocol(streams.FlowControlMixin,
 
 
 class Process:
+    stdin = ...  # type: Optional[streams.StreamWriter]
+    stdout = ...  # type: Optional[streams.StreamReader]
+    stderr = ...  # type: Optional[streams.StreamReader]
+    pid = ... # type: int
     def __init__(self,
             transport: transports.BaseTransport,
             protocol: protocols.BaseProtocol,