[QGIS-Developer] Is there a recommended way to include external Python packages

[Gerald Kogler]

I'm actually developing a QGIS plugin which depends on pysftp package to 
upload files to a server. For now I thought on asking users to install 
it manually, but it would be great to automatize the installation process.

So the options I see would be:
1. Install external package using pip from inside the plugin.
2. Include and ship external package with the plugin.
3. Ask user to install it manually using pip install.

Is there a recommended way to handle this situation?