Update tqdm to 4.31.1

[pyup-bot]

This PR updates tqdm from 4.30.0 to 4.31.1.

Changelog

4.31.0

- Write bytes to `stdout`/`stderr` in py2 (https://bugs.python.org/issue21363, 589 -> 598)
- Add `write_bytes` parameter for py2/3 default override
- support custom bar characters vias `ascii` parameter (223 -> 227)
- implement, document and test custom bar spinners
- support custom `bar_format` with unknown `total` (282 -> 284)
- fix `TypeError` when `unit_scale` and unknown `total`
- document `format_dict`
- expose `elapsed_s`, `remaining_s` to `bar_format`
- add `unit` option to `bar_format` (368)
- auto `README.rst` generation framework
- add notes in `CONTRIBUTING.md`
- update tests
- update documentation
- misc code tidy

Links

• PyPI: https://pypi.org/project/tqdm
• Changelog: https://pyup.io/changelogs/tqdm/
• Repo: https://github.com/tqdm/tqdm