switched to python:alpine docker image, added conditional call to pytest, made call to pylama conditional

.gitlab-ci.yml

@@ -1,4 +1,4 @@
-image: python
+image: python:alpine
 
 test:pylama:
     stage: test
@@ -8,5 +8,15 @@ test:pylama:
     before_script:
         - if test -r requirements.txt; then pip install -r requirements.txt; fi
     script:
-        pylama *.py
+        if test -x "$(which pylama)"; then pylama *.py; fi
+
+test:pytest:
+    stage: test
+    only:
+        - master
+        - develop
+    before_script:
+        - if test -r requirements.txt; then pip install -r requirements.txt; fi
+    script:
+        if test -x "$(which pytest)"; then pytest .; fi