fix: remove unused future dependency (#173)

As far as I can tell, we never used this dependency. We only support Python
3+, and the dependency has a known vulnerability and is not maintained.

There is not reason for us to keep it, and we were requested to remove it
in #172.

setup.py

@@ -10,7 +10,7 @@
 with open("CHANGELOG.rst") as changelog_file:
     changelog = changelog_file.read()
 
-requirements = ["future>=0.17.1", "python-dateutil>=2.7.5", "requests>=2.20"]
+requirements = ["python-dateutil>=2.7.5", "requests>=2.20"]
 
 extras_require = {"docs": ["Sphinx==1.8.1", "sphinx-rtd-theme==0.4.2"]}