Add CI to run doctests

This MR enables Gitlab CI to automatically run all doctests using pytest.

In addition, a requirements file is added to keep track of dependencies.

.gitlab-ci.yml

+pytest:
+  tags:
+    - conda
+  stage: test
+  before_script:
+    - python3 -m pip install -r requirements.txt
+    - python3 -m pip install pytest
+  script:
+    - pytest --doctest-modules --junitxml=report.xml
+  artifacts:
+    when: always
+    reports:
+      junit: report.xml