GitLab: Add basic CI (build stage)

1 job for dev/addci in 1 minute and 4 seconds (queued for 5 minutes and 15 seconds)
Name Stage Failure
failed
job Build
Successfully installed bottle-0.12.19 certifi-2020.12.5 chardet-4.0.0 click-7.1.2 colorama-0.4.4 idna-2.10 marshmallow-2.21.0 platformio-5.1.1 pyelftools-0.27 pyserial-3.5 requests-2.25.1 semantic-version-2.8.5 tabulate-0.8.9 urllib3-1.26.4
WARNING: You are using pip version 20.0.2; however, version 20.3.4 is available.
You should consider upgrading via the '/usr/local/bin/python -m pip install --upgrade pip' command.
$ platformio run
Error: Python 3.6 or later is required for this operation.
Please check a migration guide:
https://docs.platformio.org/en/latest/core/migration.html#drop-support-for-python-2-and-3-5
Cleaning up file based variables
ERROR: Job failed: exit code 1