Issue #21906: Make Tools/scripts/md5sum.py work in Python 3.

Patch by Zachary Ware.
asyncio, Tulip issue 180: Make Server attributes and methods private

- loop, waiters and active_count attributes are now private
- attach(), detach() and wakeup() methods are now private

The sockets attribute remains public.
asyncio: improve the documentation of servers

- Fix the documentation of Server.close(): it closes sockets
- Replace AbstractServer with Server
- Document Server.sockets attribute
asyncio: sync with Tulip

* Tulip issue #183: log socket events in debug mode

  - Log most important socket events: socket connected, new client, connection
    reset or closed by peer (EOF), etc.
  - Log time elapsed in DNS resolution (getaddrinfo)
  - Log pause/resume reading
  - Log time of SSL handshake
  - Log SSL handshake errors
  - Add a __repr__() method to many classes

* Fix ProactorEventLoop() in debug mode. ProactorEventLoop._make_self_pipe()
  doesn't call call_soon() directly because it checks for the current loop
  which fails, because the method is called to build the event loop.

* Cleanup _ProactorReadPipeTransport constructor. Not need to set again
  _read_fut attribute to None, it is already done in the base class.
asyncio: Server.close() leaves client sockets open