Add a .coveragerc file to fix coverage for playwright

Playwright uses greenlet, which requires concurrency=greenlet for
coverage
microsoft/playwright-python#313

But we can't set it in pyproject.toml as an option for pytest-cov,
we need to put it in a .coveragerc
pytest-dev/pytest-cov#303

.coveragerc

@@ -0,0 +1,2 @@
+[run]
+concurrency = greenlet