Update dependencies and test to drop python 3.5 support.

The reason for dropping python 3.5 support is that it is not compatible
with the feedparser dependency. The version is anyway not supported any
more (security fixes stopped on 2020/09).

.travis.yml

@@ -21,11 +21,6 @@ matrix:
         - source env/bin/activate
 
     # Mock tests (with mock API)
-    - os: linux
-      language: python
-      python: 3.5
-      env:
-        - TO_TEST=TEST_MOCK
     - os: linux
       language: python
       python: 3.6

setup.py

@@ -45,16 +45,15 @@ setup(
         },
 
     include_package_data=True,
-
     install_requires=['pyyaml', 'bibtexparser>=1.0', 'python-dateutil', 'six',
                       'requests', 'configobj', 'beautifulsoup4', 'feedparser'],
     extras_require={'autocompletion': ['argcomplete'],
                     },
 
+    python_requires='>=3.6',
     classifiers=[
         'Development Status :: 4 - Beta',
         'License :: OSI Approved :: GNU Lesser General Public License v3 (LGPLv3)',
-        'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
         'Intended Audience :: Developers',
         'Intended Audience :: Science/Research',