Remove hotfix specifying of six dependency

Author: robertdstein

.travis.yml

@@ -1,7 +1,7 @@
 language: python
 install:
 - pip install --upgrade pip
-- pip install -e . --use-deprecated=legacy-resolver
+- pip install -e .
 matrix:
   include:
   - python: 3.7

setup.py

@@ -34,8 +34,7 @@
         "astropy==4.2.1",
         "sphinx==4.0.1",
         "jupyter==1.0.0",
-        "coveralls==3.0.1",
-        "six==1.16.0"
+        "coveralls==3.0.1"
     ],
     package_data={'flarestack': [
         'data/public/icecube/all_sky_point_source/raw_data/3year-data-release.zip']},