Add façade function for creating python api sessions.

Add a utility function to make creating python api session more comfortable.
This was suggested at neovim/neovim#1373.

For example, instead of:
```
from neovim import socket_session, Nvim
session = tcp_session(address=<address>, port=<port>)
nvim = Nvim.from_session(session)
```
you can do now:
```
from neovim import attach
nvim = attach('tcp', address=<address>, port=<port>)
```