Phono3py

- Maintenance release including small bug fixes.

- Maintenance release to follow the change of phonopy at v2.12.1.
- Installation using `setup.py` now requires creating `site.cfg` file. See
<https://phonopy.github.io/phono3py/install.html> and
[PR 59](https://github.com/phonopy/phono3py/pull/59).
- Dorp python 3.6 support, and dependencies of numpy and matplotlib versions are
updated:

- Python >= 3.7
- numpy >= 1.15.0
- matplotlib >= 2.2.2

- Fix of a critical bung in the direct solution. See the detail as commit log of
[54d4ddab](https://github.com/phonopy/phono3py/commit/54d4ddab6f3fbf9435bdfe8b27757be1d5c4ebf6).
- Aiming modernizing phono3py code, required python version and package versions
were changed to

- Python >= 3.6
- numpy >= 1.11
- matplotlib >= 2.0

- For developers, flake8, black, pydocstyle, and isort were introduced. See
`REAEME.md` and `.pre-commit-config.yaml`.

This is a major version release. There are some backward-incompatible changes.

1. Grid point indexing system to address grid points of q-points is changed.
2. Array data types of most of the integer arrays are changed to `dtype='int_'`
from `dtype='intc'`.
3. Python 3.5 or later is required.

To emulate the version 1.x behaviour in `phono3py` command, try `--v1` option.
To emurate the version 1.x behaviour in API, specify `store_dense_gp_map=False`
and `store_dense_svecs=False` in instatiation of `Phono3py` class or phon3py
loader.

- Fix `--read-gamma` option to work.

- Fix PyPI source distribution package