Installing unicode_slugify does not work on Ubuntu 18.04

[raboof]

When I try to build opensnitch from source on Ubuntu 18.04, it fails with:

Collecting unicode_slugify==0.1.3 (from -r requirements.txt (line 3))
  Using cached https://files.pythonhosted.org/packages/8c/ba/1a05f61c7fd72df85ae4dc1c7967a3e5a4b6c61f016e794bc7f09b2597c0/unicode-slugify-0.1.3.tar.gz
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-build-y8uwaimx/unicode-slugify/setup.py", line 7, in <module>
        long_description=open('README.md').read(),
      File "/usr/lib/python3.6/encodings/ascii.py", line 2
6, in decode
        return codecs.ascii_decode(input, self.errors)[0]
    UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 361: ordinal not in range(128)

I see this is also reported upstream at mozilla/unicode-slugify#39