arrowbricks v5.2.1

• Fix an async token_provider failing with AuthError: RuntimeError: no running event loop on a client that had not run any query yet.
  stream_query_json was the usual trigger: since 3.2.0 its statement is
  submitted from a background task, and the provider used to look for its
  event loop in whichever task first asked for a token. Once any
  execute() had run on the client, the loop it found then was reused,
  which is why it only hit cold clients.
• The event loop (and contextvars) is now captured when you call the
  method, where your loop is running, and travels with the query into
  every background task it starts: the submit, chunk downloads, the Thrift
  fetch loop, and best-effort cancels. Each token request therefore runs on
  the loop of the query that needs it, also when one client is used from
  several loops at once (one per thread) -- before, a token for one loop's
  query could be fetched on another loop.
• Sync token_providers are unchanged. An async on_event callback picks
  its event loop the same way now, so it also fires for a cold client's
  first query when that query times out or is cancelled (it used to need an
  earlier query on the same client to have reported first).
• Tests: tests/test_token_provider_event_loop.py (both protocols; every
  public entry point on a cold client, two successive asyncio.run()s,
  asyncio.run() in a thread, two threads at once, and a timed-out stream
  whose cancel and async on_event must still run),
  tests_py/test_token_provider.py, and call-context tests in
  call_context.rs, wiremock_pipeline.rs and wiremock_thrift.rs. The
  cold-client streaming tests fail on 5.2.0.
• No API changes.