Allow collecting coverage without `pytest-cov`

[flying-sheep]

vscode-python assumes that pytest-cov is required to collect coverage. Many people just run coverage run pytest instead, which has several advantages.

So you should

1. have the “run with coverage” button check if if pytest-cov is installed in the environment, and if not, run coverage.Coverage().start() before pytest.main() instead of adding --cov* args to pytest.main() (in run_pytest_script.py)
2. in the plugin, you need to do a different check than checking for --cov-branch to set INCLUDE_BRANCHES