Pyzstd

- Add two convenient functions: [`compress_stream()`](https://pyzstd.readthedocs.io/#compress_stream) and [`decompress_stream()`](https://pyzstd.readthedocs.io/#decompress_stream).
- Some improvements.

- Upgrade zstd source code from v1.4.5 to [v1.4.8](https://github.com/facebook/zstd/releases/tag/v1.4.8).
- v1.4.6 is a non-public release for Linux kernel.
- v1.4.8 is a hotfix for [v1.4.7](https://github.com/facebook/zstd/releases/tag/v1.4.7).
- Some improvements, no API changes.

- `ZstdDecompressor` class: now it has the same API and behavior as BZ2Decompressor / LZMADecompressor classes in Python standard library, it stops after a frame is decompressed.
- Add an `EndlessZstdDecompressor` class, it accepts multiple concatenated frames. It is renamed from previous `ZstdDecompressor` class, but `.at_frame_edge` is `True` when both the input and output streams are at a frame edge.
- Rename `zstd_open()` function to `open()`, consistent with Python standard library.
- `decompress()` function:
- ~9% faster when: there is one frame, and the decompressed size was recorded in frame header.
- raises ZstdError when input **or** output data is not at a frame edge. Previously, it only raise for output data is not at a frame edge.

- No longer use [Argument Clinic](https://docs.python.org/3/howto/clinic.html), now supports Python 3.5+, previously 3.7+.

- It seems the API is stable.

- The first version upload to PyPI.
- Includes zstd [v1.4.5](https://github.com/facebook/zstd/releases/tag/v1.4.5) source code.