add pylint run to CI

.gitlab-ci.yml

 before_script:
-  - sudo apt-get update -qq && sudo apt-get install -y -qq libtool pkg-config python3-openssl gir1.2-gtk-3.0 python3-gi python3-nbxmpp-nightly
+  - sudo apt-get update -qq && sudo apt-get install -y -qq libtool pkg-config python3-openssl gir1.2-gtk-3.0 python3-gi python3-nbxmpp-nightly pylint3
   - sudo apt-get build-dep -y -qq gajim-default-nightly
 
 stages:
@@ -12,6 +12,11 @@ run-test:
     - ./autogen.sh
     - make test_nogui
 
+run-pylint:
+  stage: test
+  script:
+    - pylint3 --additional-builtins=_ --disable=all --enable=E0001,E0011,E0012,E0100,E0101,E0102,E0103,E0104,E0105,E0106,E0107,E0108,E0202,E0221,E0222,E0235,E0501,E0502,E0503,E0602,E0603,E0604,E0701,E0702,E1001,E1002,E1003,E1004,E1111,E1120,E1121,E1122,E1123,E1124,E1125,E1200,E1201,E1205,E1206,E1300,E1301,E1302,E1303,E1304,E1305,E1306,E1310,E1700,E1701 gajim
+
 run-build:
   stage: build
   script: