use python 3.7 and freeze the pip packages

.gitlab-ci.yml

+---
 # -*- coding: utf-8 -*-
 
-image: python:3.6
+image: python:3.7
 
 variables:
   PIP_CACHE_DIR: "$CI_PROJECT_DIR/pip-cache"

requirements.txt

-pelican
-Markdown
-icalendar
-Pillow
+blinker==1.4
+docutils==0.14
+feedgenerator==1.9
+icalendar==4.0.3
+Jinja2==2.10
+Markdown==3.0.1
+MarkupSafe==1.1.0
+pelican==4.0.1
+Pillow==5.4.1
+Pygments==2.3.1
+python-dateutil==2.7.5
+pytz==2018.9
+six==1.12.0
+Unidecode==1.0.23