sync.WorkGroup: Provide "with" support

So that it becomes possible to write

    with WorkGroup(ctx) as wg:
        wg.go(f1)
        wg.go(f2)

instead of

    wg = WorkGroup(ctx)
    defer(wg.wait)
    wg.go(f1)
    wg.go(f2)

or

    wg = WorkGroup(ctx)
    wg.go(f1)
    wg.go(f2)
    wg.wait()

This is sometimes handy and is referred to as "structured concurrency"
in Python world.

sync.Sema, sync.Mutex, sync.RWMutex already support "with".
sync.WaitGroup is imho too low-level, but we might consider adding
"with" support for it in the future as well.

In general pygolang way is to use defer instead of plugging all classes
with __enter__/__exit__ "with" support, but for small well-known class of
concurrency-related things its seems "with" support is worth it:

- having "with" for sync.Mutex+co allows it to be used as a drop-in
  replacement instead of threading.Lock+co, and
- having "with" for sync.WorkGroup - the most commonly-used tool to
  spawn jobs and wait for their completion - makes it on-par with
  "structured concurrency".

/reviewed-on nexedi/pygolang!12