ftplib-dev software package provides virtual package provided by libftp-dev, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install ftplib-dev
ftplib-dev is installed in your system.
Make ensure the ftplib-dev package were installed using the commands given below,
$ sudo dpkg-query -l | grep ftplib-dev*
You will get with ftplib-dev package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Virtual packages |
| Package | ftplib-dev |
| Version | |
| Description | virtual package provided by libftp-dev |
| Sub Section | Virtual |