test: unblock pre-push hook collection

- pyproject.toml: norecursedirs for tests/playwright-upstream/, a vendored
  Microsoft Playwright test suite with its own pixelmatch API version
  mismatch. We run it explicitly when doing compat audits, not on every
  push. Default collection now ignores it so the pre-push hook (which
  runs the full default pytest collection) doesn't error out.
- tests/test_service_worker.py: replace em-dash with hyphen inside a
  bytes literal at line 91. Python rejects non-ASCII bytes literals
  with SyntaxError at collection time. Now collects cleanly.

Both were blocking unrelated pushes (e.g. the issue #18 fix in the
previous commit). Splitting them out so the issue #18 commit stays
focused.

pyproject.toml

@@ -39,6 +39,12 @@ markers = [
     "linux_only: tests that require Linux platform",
 ]
 addopts = "-m 'not slow and not e2e'"
+# tests/playwright-upstream/ is a vendored Microsoft Playwright test suite
+# used for compatibility verification on demand. It has its own deps
+# (pixelmatch with API not matching our version) and a conftest that fails
+# collection in our env. Run it explicitly with --override-ini for compat
+# audits, not on every push.
+norecursedirs = ["playwright-upstream"]
 
 [project.scripts]
 invisible-playwright = "invisible_playwright.cli:main"