Do not install Python packages in GitLab CI jobs

As Python QA tools, BIND system test prerequisites, and documentation
building utilities are now all included in operating system images used
in GitLab CI, do not use pip for installing them in each CI job any
more.

(cherry picked from commit e3c217296d)

.gitlab-ci.yml

@@ -195,7 +195,6 @@ stages:
   before_script:
     - test -w "${CCACHE_DIR}" && export PATH="/usr/lib/ccache:${PATH}"
     - test -n "${OOT_BUILD_WORKSPACE}" && mkdir "${OOT_BUILD_WORKSPACE}" && cd "${OOT_BUILD_WORKSPACE}"
-    - pip3 install pytest requests || pip install pytest requests || true
   script:
     - *configure
     - make depend | grep "error:" && exit 1
@@ -254,7 +253,6 @@ stages:
   before_script:
     - *setup_interfaces
     - *setup_softhsm
-    - pip3 install pytest requests || pip install pytest requests || true
   script:
     - ( cd bin/tests/system && make -j${TEST_PARALLEL_JOBS:-1} -k test V=1 )
     - test -s bin/tests/system/systests.output
@@ -431,8 +429,6 @@ flake8:
   needs:
     - job: autoreconf
       artifacts: true
-  before_script:
-    - pip3 install flake8
   script:
     - *configure
     - flake8 --max-line-length=80 $(git ls-files '*.py' | grep -v 'ans\.py')
@@ -446,12 +442,9 @@ pylint:
   needs:
     - job: autoreconf
       artifacts: true
-  before_script:
-    - pip3 install pylint
-    - PYTHONPATH="$PYTHONPATH:$CI_PROJECT_DIR/bin/python"
-    - pip3 install pytest requests || pip install pytest requests
   script:
     - *configure
+    - PYTHONPATH="$PYTHONPATH:$CI_PROJECT_DIR/bin/python"
     - pylint --rcfile $CI_PROJECT_DIR/.pylintrc $(git ls-files '*.py' | grep -v 'ans\.py')
   only:
     - merge_requests