build: Lint install_requires in setup.py

While doing packaging works I noticed that
pypi file 'tuf.egg-info/requires.txt' was modified on build.

Let's sort dependencies alphabetically
to avoid lint to change sources files.

Signed-off-by: Philippe Coval <rzr@users.sf.net>

setup.py

@@ -113,8 +113,8 @@
  python_requires="~=3.6",
  install_requires = [
  'requests>=2.19.1',
- 'six>=1.11.0',
- 'securesystemslib>=0.18.0'
+ 'securesystemslib>=0.18.0',
+ 'six>=1.11.0'
  ],
  packages = find_packages(exclude=['tests']),
  scripts = [