Uses a local .venv in GitLab CI to match the Makefile

This solves a CI failure on the make docs step.

To test, I ran all the following commands found in .gitlab-ci.yml:

• python3 -m venv .venv
• source .venv/bin/activate
• python3 -m pip -V
• python3 -m pip install --upgrade pip
• python3 -m pip install -r requirements-dev.txt
• python3 -m pip install ansible
• python3 -m pip install ansible-core
• .venv/bin/ansible --version

As well as .venv/bin/ansible-playbook --version.

Closes #50 (closed)