When creating a sub-process using `asyncio.create_subprocess_exec()`,it returns a `Process` instance that has a `stdout` property. Thisproperty is intended to be an asyncio version of the `stdout` propertyof the `Popen` instance from the `subprocess` module.An important aspect of `Popen.stdout` property is that you can closeit. This is a signal to the sub-process that is generating output thatit should cleanly terminate. This is a common pattern in processesused in shell pipelines. Indeed, the object located at `Popen.stdout`has a `close()` method. This pattern is demonstrated below:```pythonimport subprocessproc = subprocess.Popen(["yes"], stdout=subprocess.PIPE) # start subprocessdata = proc.stdout.read(4096) # get dataproc.stdout.close() # signal to process to cleanly shutdownproc.wait() # wait for shutdown```Unfortunately this pattern cannot be reproduced easily with the`stdout` property of the `Process` instance returned from`asyncio.create_subprocess_exec()` because `stdout` is an instance of`StreamReader` which does not have the `close()` method.This change adds a `close()` method to the `StreamReader` class sothat asyncio version of the `subprocess` module may support thispattern of managing sub-processes. This change is consistent with theasyncio ecosystem as the companion `StreamWriter` class already has a`close()` method, along with other methods that expose its inner"transport" object. It's also trivial to implement, since it'sessentially a wrapper method around the inner transport object's`close()` method.