Pynvim

Cleanup after the `pynvim` rename, as well as a few bugfixes and
improvements.

This release renames the the package name (for PyPI and importing) to `pynvim`.
`import neovim` is still supported as an alias for backwards compatibility, but new API users should use `import pynvim`.

Unfotunately, due to limitations in `pip`, the `neovim` package cannot be safely upgraded with `pip install --upgrade neovim` on all systems. The safest option to upgrade is

`pip uninstall neovim`
`pip uninstall pynvim only if you tried to upgrade already and it failed`
`pip install pynvim`

At this point, it is safe to `pip install neovim` again, if any third-party package depends on it. Otherwise it shouldn't be necessary.

For a new install, it is enough to use

`pip install pynvim`

This release is otherwise functionally identical to pynvim 0.3.0, except for an improved error message.

Requires Nvim 0.3.0 or later.

This is a bugfix release.

This release adds support for Python 3.7 which now is in beta. `async` is a keyword in python3.7, and it should be replaced with `async_` when used as a keyword argument to API methods. `async` is still supported with python 2.7 and 3.6 for the moment, but considered deprecated.

Also, integration with the in-process lua interpreter in Nvim is now supported. Please see the [documentation](http://pynvim.readthedocs.io/en/latest/usage/python-plugin-api.html#lua-integration) for usage instructions.