Require 'neovim' instead of 'pynvim'

The 'neovim' package acts as transition package for 'pynvim'.

This is solely for backward compatibility for operating systems that provide the
'neovim' package via their own package format. E.g. Debian stable (Buster)
provides a 'python3-neovim' package, which provides the 'neovim' Python package,
but not 'pynvim'.

If you install both through pip3, feel free to remove the transitional 'neovim'
package after installing or upgrading 'neovim-remote'.

setup.py

@@ -15,7 +15,7 @@
     long_description = long_description,
     long_description_content_type = 'text/markdown',
     python_requires  = '>=3.4',
-    install_requires = ['pynvim', 'psutil', 'setuptools'],
+    install_requires = ['neovim', 'psutil', 'setuptools'],
     entry_points     = {
         'console_scripts': ['nvr = nvr.nvr:main']
     },