Enable more pylint checks and test also against Python 3.8

.travis.yml

@@ -12,7 +12,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 os: linux
-dist: xenial
+dist: bionic
 sudo: false
 language: python
 cache: pip
@@ -21,6 +21,7 @@ python:
   - "3.5"
   - "3.6"
   - "3.7"
+  - "3.8"
 
 install: pip install tox-travis
 script: tox

tox.ini

@@ -15,7 +15,7 @@
 skip_missing_interpreters = true
 skipsdist = true
 envlist =
-    py35,py36,py37
+    py35,py36,py37,py38
 
 geany_modules = geany latest_version news nightlybuilds pastebin static_docs
 
@@ -53,7 +53,15 @@ skip = local_settings.py,local_settings.docker.py
 ignore=.git
 ignore-patterns=local_settings.py,local_settings.docker.py
 persistent=no
-load-plugins=pylint_django
+load-plugins=
+    pylint_django,
+    pylint.extensions.bad_builtin,
+    pylint.extensions.check_elif,
+    pylint.extensions.comparetozero,
+    pylint.extensions.emptystring,
+    pylint.extensions.mccabe,
+    pylint.extensions.overlapping_exceptions,
+    pylint.extensions.redefined_variable_type
 
 [MESSAGES CONTROL]
 disable=