feat: add .env support

For #2 (closed)

What does this MR do?

Adds some .env support using python-dotenv. Fairly self explanatory, but for the uninitiated:

• you can provide your secrets / environment variables in a .env file. Don't commit the .env to the repo.
• when running the ucam-gitlab-stats API, you can either provide the token as an env var, or put it in the .env.

Checks

Added a .env file with GITLAB_TOKEN=... then ran the following locally:

poetry run ucam-gitlab-stats --labels "team::Identity" --print-iterations --week-count=13 14

All worked as expected