tqdm v4.51.0 stable

• add {eta} datetime bar_format argument (#1055 <- #1051)
  • e.g. bar_format='{l_bar}{bar}| {n_fmt}/{total_fmt} [{rate_fmt} ETA:{eta:%y-%m-%d %H:%M}{postfix}]'
• fix py3 CLI --update & --update_to
• replace nosetests with pytest (#1052, #1045)
• add & update tests