Pyzstd

- Upgrade zstd source code from v1.5.5 to [v1.5.6](https://github.com/facebook/zstd/releases/tag/v1.5.6)
- Fix pyzstd_pep517 parameter name in `get_requires_for_build_wheel`
- Deprecate support for Python version before 3.8 and stop building wheels for them
- Minor fixes in type hints
- Refactor README & CHANGELOG files

- Fix `SeekableZstdFile` class can't open new file in appending mode.
- Support sub-interpreter on CPython 3.12+, can utilize [per-interpreter GIL](https://docs.python.org/3.12/whatsnew/3.12.html#pep-684-a-per-interpreter-gil).
- On CPython(3.5~3.12)+Linux, use another output buffer code that can utilize the `mremap` mechanism.
- Change repository URL and maintainer following the deletion of the GitHub account of the original author, Ma Lin (animalize). See [1](https://github.com/Rogdham/pyzstd/issues/1).

ZstdFile class related changes:

- Add [`SeekableZstdFile`](https://pyzstd.readthedocs.io/#SeekableZstdFile) class, it's a subclass of `ZstdFile`, supports [Zstandard Seekable Format](https://github.com/facebook/zstd/blob/dev/contrib/seekable_format/zstd_seekable_compression_format.md).
- Add _mode_ argument to `ZstdFile.flush()` method, now it can flush a zstd frame.
- Add _read_size_ and _write_size_ arguments to `ZstdFile.__init__()` method, can work with Network File Systems better.
- Optimize `ZstdFile` performance to C language level.

ZstdDict class changes:

- Fix these advanced compression parameters may be ignored when loading a dictionary: `windowLog`, `hashLog`, `chainLog`, `searchLog`, `minMatch`, `targetLength`, `strategy`, `enableLongDistanceMatching`, `ldmHashLog`, `ldmMinMatch`, `ldmBucketSizeLog`, `ldmHashRateLog`, and some non-public parameters.
- When compressing, load undigested dictionary instead of digested dictionary by default. Loading again an undigested is slower, see [differences](https://pyzstd.readthedocs.io/#ZstdDict.as_digested_dict).
- Add [`.as_prefix`](https://pyzstd.readthedocs.io/#ZstdDict.as_prefix) attribute. Can use zstd as a [patching engine](https://pyzstd.readthedocs.io/#patching-engine).

- Upgrade zstd source code from v1.5.4 to [v1.5.5](https://github.com/facebook/zstd/releases/tag/v1.5.5).

- Upgrade zstd source code from v1.5.2 to [v1.5.4](https://github.com/facebook/zstd/releases/tag/v1.5.4). v1.5.3 is a non-public release.
- Support `pyproject.toml` build mechanism (PEP-517). Note that specifying build options in old way may be invalid, see [build commands](https://pyzstd.readthedocs.io/#build-pyzstd).
- Support "multi-phase initialization" (PEP-489) on CPython 3.11+, can work with CPython sub-interpreters in the future. Currently this build option is disabled by default.
- Add a command line interface (CLI).