request.py: Added SimpleRequestCondition, a Request-compatible condition variable with a subset of the features from threading.Condition.
See also: e6fda6988c2c80e721b301506fbd479e3a90b312 and 6461312181088508196b11ac84d4c90b6bc1bdf1.
It turns out that threading.Condition(RequestLock()) does NOT suffice when the wait()ing thread is a worker thread.
docs: Updated request docs to include SimpleRequestCondition.
docs: Built.