Make dynamic_ncols=True the default

Any reason why `dynamic_ncols` is not True by default?

- [X] I have visited the [source website], and in particular
  read the [known issues]
- [X] I have searched through the [issue tracker] for duplicates

  [source website]: https://github.com/tqdm/tqdm/
  [known issues]: https://github.com/tqdm/tqdm/#help
  [issue tracker]: https://github.com/tqdm/tqdm/issues?q=
