fmgapi is not python 3.5 compatible (it uses python 3.7 f-string)

[jepafortinet]

During install:

root@lxc:~/ftntlib-master# pip3 install setuptools
...
root@lxc:~/ftntlib-master# python3 setup.py install
running install
...

byte-compiling build/bdist.linux-x86_64/egg/ftntlib/fmgapi.py to fmgapi.cpython-35.pyc
  File "build/bdist.linux-x86_64/egg/ftntlib/fmgapi.py", line 57
    self._url = f'{protocol}://{host}:{port}/jsonrpc'
                                                    ^
SyntaxError: invalid syntax
...