Replace Python 2.7 workaround in 'bidi.BackgroundConsumer._thread_main' with 'Condition.wait_for'

[tseaver]

See this note.

Blocked on merge of PR #212, dropping Python 2.7 support.

[tswast]

Just because I had trouble finding this with a Google search, I think these are the relevant docs for the Condition.wait_for method.