Biglist

- Bug fixes.
- Simplified bookkeeping for 'concurrent_iter'.

- Upgrade for `upathlib` 0.6.4, which has a breaking change about serializers.

- Minor changes to prepare for `upathlib` upgrade.
- Make Orjson-based storage formats optional.
- Dependency on `upathlib` specifies default `upathlib` only.
User who needs enhanced `upathlib` can simply install that explicitly
prior to installing `biglist`, e.g. `pip install upathlib[gcs]`.

- Change default format from 'orjson' to 'pickle'.

- Minor refinements and speed improvements.
- Explicitly separate concurrent (multi-worker) and non-concurrent
(uni-worker) writing, so as to improve speed in the non-concurrent case.
- New methods `concurrent_append`, `concurrent_extend`.
- Renamings:
- `reset_file_iter` -> `new_concurrent_iter`
- `iter_files` -> `concurrent_iter`
- `file_iter_stat` -> `concurrent_iter_stat`
- Refactor and simplify test and build processes.

- Fix and fine-tuning related to threading.