Clean up CI spec

.gitlab-ci.yml

 image: python:alpine
 
-before_script:
-   - pip install -e .
-   - pip install pylint pycodestyle pydocstyle
-
-pylint:
  stage: test
- script: 
+ script:
+   - pip install pylint
    - pylint pydtn examples
-   
+
 pycodestyle:
  stage: test
- script: 
+ script:
+   - pip install pycodestyle
    - pycodestyle pydtn examples
- 
+
 pydocstyle:
   stage: test
   script:
+    - pip install pydocstyle
     - pydocstyle pydtn examples