test: add pytest-timeout and pin the Windows selector event loop

A hung test could wedge the whole run: aiortc's ICE stack never completes
a loopback DataChannel handshake on Windows' default ProactorEventLoop, and
nothing capped it. Two changes, both no-ops off Windows:

- `timeout = 60` in the root pytest config (+ pytest-timeout in the dev
  extras) so a stall fails the test instead of the suite.
- a root conftest that selects WindowsSelectorEventLoopPolicy on win32 only,
  which is what aiortc needs there. Trade-off, documented: the selector loop
  cannot spawn subprocesses on Windows, so the ffmpeg streaming tests fail
  there rather than pass -- they need a per-module override or skipif(win32).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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