Fix #173 - Require gym-pusht to be installed for test_examples_3_and_2

[aliberts]

What this does

Fixes #173

How it was tested

I've added a pytest-minimal job in CI to check that tests are passing with no envs installed: successful job

How to checkout & try? (for the reviewer)

Checkout this branch and run

poetry install --sync --extras "test"
DATA_DIR="tests/data" python -m pytest -v ./tests

[AshisGhosh]

Confirmed to be working:

===================================================== 39 passed, 26 skipped in 16.65s ======================================================