**Implemented enhancements:**
- Include pre-built C sources and support both pre-built source installation and
native Cython compile (cython is no longer mandatory dependency).
- Check compatible python versions. (3.4 and 3.7 additionally)
- Scipy is no longer required.
**Fixed bugs:**
- Omit `functools.partialmethod` to keep python2 compatibility.
- Fix minor argparse issues and a error message.