Add linting to the CI

.gitlab-ci.yml

 image: debian
 
-test:
+stages:
+  - linting
+  - test
+
+bandit:
+  stage: linting
+  script:
+  - apt-get -qqy update
+  - apt-get -qqy install --no-install-recommends python3-bandit
+  - bandit -r ./src --format txt --skip B404,B603
+
+pyflakes:
+  stage: linting
+  script:
+  - apt-get -qqy update
+  - apt-get -qqy install --no-install-recommends pyflakes3
+  - pyflakes3 ./src
+
+tests:
+  stage: test
   script:
   - apt-get -qqy update
   - apt-get -qqy install --no-install-recommends python3-mutagen python3-gi-cairo gir1.2-poppler-0.18 gir1.2-gdkpixbuf-2.0 libimage-exiftool-perl python3-coverage