diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index bdb2c7f4206e55a535eaf638272f2feca4d33bf8..f48d0fe07cf949be37018cd787d1a30f39009e52 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,6 +1,6 @@
 # Official language image. Look for the different tagged releases at:
 # https://hub.docker.com/r/library/python/tags/
-image: ${CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX}/python:3.6
+image: ${CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX}/python:latest
 
 # Change pip's cache directory to be inside the project directory since we can
 # only cache local items.
@@ -15,7 +15,6 @@ variables:
 cache:
   paths:
     - .cache/pip
-    - venv/
 
 before_script:
   - pip install virtualenv
@@ -41,6 +40,63 @@ unit-test-job:
     reports:
       junit: nose2-junit.xml
 
+unit-test-2.7-job:
+  image: ${CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX}/python:2.7
+  stage: test
+  script:
+    - make test
+
+unit-test-3.7-job:
+  image: ${CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX}/python:3.7
+  stage: test
+  script:
+    - make test 2>&1 | tee errors-3.7.log
+  artifacts:
+    when: always
+    paths:
+      - errors-3.7.log
+
+unit-test-3.8-job:
+  image: ${CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX}/python:3.8
+  stage: test
+  script:
+    - make test 2>&1 | tee errors-3.8.log
+  artifacts:
+    when: always
+    paths:
+      - errors-3.8.log
+
+unit-test-3.9-job:
+  image: ${CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX}/python:3.9
+  stage: test
+  script:
+    - make test 2>&1 | tee errors-3.9.log
+  artifacts:
+    when: always
+    paths:
+      - errors-3.9.log
+
+check-deprecation-warnings-3.7:
+  before_script: []
+  stage: check-warnings
+  script:
+    - "if [[ $(grep DeprecationWarning errors-3.7.log) ]]; then cat errors-3.7.log; exit 1; fi"
+  allow_failure: true
+
+check-deprecation-warnings-3.8:
+  before_script: []
+  stage: check-warnings
+  script:
+    - "if [[ $(grep DeprecationWarning errors-3.8.log) ]]; then cat errors-3.8.log; exit 1; fi"
+  allow_failure: true
+
+check-deprecation-warnings-3.9:
+  before_script: []
+  stage: check-warnings
+  script:
+    - "if [[ $(grep DeprecationWarning errors-3.9.log) ]]; then cat errors-3.9.log; exit 1; fi"
+  allow_failure: true
+
 pylint-test-job:
   stage: test
   script:
diff --git a/Makefile b/Makefile
index a991c470963fdb3b77fe3f570418ecd73003066f..562c2bf30ed3804ebace9e29d1d0c51a3749a1f7 100644
--- a/Makefile
+++ b/Makefile
@@ -99,7 +99,7 @@ help:
 
 
 show-version: FORCE
-	@PYTHONPATH=lib python3 -c "import idea; print(idea.__version__);"
+	@PYTHONPATH=lib python -c "import idea; print(idea.__version__);"
 
 
 #-------------------------------------------------------------------------------
@@ -109,12 +109,12 @@ deps: deps-python deps-python-dev
 
 deps-python-dev: FORCE
 	@echo "\n$(GREEN)*** Installing Python development dependencies ***$(NC)\n"
-	@pip3 install -r requirements-dev.pip
+	@pip install --no-cache-dir -r requirements-dev.pip
 
 
 deps-python: FORCE
 	@echo "\n${GREEN}*** Installing Python dependencies ***${NC}\n"
-	@pip3 install -r requirements.pip --upgrade
+	@pip install --no-cache-dir -r requirements.pip --upgrade
 
 
 #-------------------------------------------------------------------------------
@@ -138,11 +138,11 @@ pyflakes:
 
 pyflakes-lib: FORCE
 	@echo "\n${GREEN}*** Checking code with pyflakes ***${NC}\n"
-	-@python3 -m pyflakes $(DIR_LIB)/*.py
+	-@python -m pyflakes $(DIR_LIB)/*.py
 
 pyflakes-test: FORCE
 	@echo "\n${GREEN}*** Checking test files with pyflakes ***${NC}\n"
-	-@python3 -m pyflakes bench_idea.py test_idea.py
+	-@python -m pyflakes bench_idea.py test_idea.py
 
 #pylint: pylint-lib pylint-test
 pylint:
@@ -150,15 +150,15 @@ pylint:
 
 pylint-lib: FORCE
 	@echo "\n${GREEN}*** Checking code with pylint ***${NC}\n"
-	-@python3 -m pylint $(DIR_LIB)/*.py --rcfile .pylintrc-lib
+	-@python -m pylint $(DIR_LIB)/*.py --rcfile .pylintrc-lib
 
 pylint-test: FORCE
 	@echo "\n${GREEN}*** Checking test files with pylint ***${NC}\n"
-	-@python3 -m pylint bench_idea.py test_idea.py --rcfile .pylintrc-test
+	-@python -m pylint bench_idea.py test_idea.py --rcfile .pylintrc-test
 
 test: FORCE
 	@echo "\n${GREEN}*** Checking code with nosetests ***${NC}\n"
-	@python3 -W always::DeprecationWarning -m nose2 --junit-xml
+	@python -W always::DeprecationWarning -m nose2 --junit-xml
 
 
 #-------------------------------------------------------------------------------
@@ -172,11 +172,11 @@ archive: FORCE
 
 bdist: FORCE
 	@echo "\n${GREEN}*** Building Python packages ***${NC}\n"
-	@python3 setup.py sdist bdist_wheel --universal
+	@python setup.py sdist bdist_wheel --universal
 
 install: FORCE
 	@echo "\n${GREEN}*** Performing local installation ***${NC}\n"
-	@pip3 install dist/idea*.whl --upgrade
+	@pip install dist/idea*.whl --upgrade
 
 deploy: FORCE
 	@echo "\n${GREEN}*** Deploying packages to PyPI ***${NC}\n"
diff --git a/requirements-dev.pip b/requirements-dev.pip
index 0ce119ace62f0f73a3d428644956f1f19d7710e6..76a46982e9f92974f42c943cb0caf64f2df33eb2 100644
--- a/requirements-dev.pip
+++ b/requirements-dev.pip
@@ -4,8 +4,8 @@ setuptools
 wheel
 twine
 docutils<0.18
-nose2==0.11.0
-pyflakes==2.1.0
-pylint==2.2.2
-sphinx==1.8.4
-sphinx-rtd-theme==0.4.2
+nose2
+pyflakes
+pylint
+sphinx
+sphinx-rtd-theme