Log Message:
py-trio: updated to 0.20.0

Trio 0.20.0 (2022-02-21)
------------------------

Features
- You can now conveniently spawn a child process in a background task
  and interact it with on the fly using ``process = await
  nursery.start(run_process, ...)``. See `run_process` for more details.
  We recommend most users switch to this new API. Also note that:

  - ``trio.open_process`` has been deprecated in favor of
    `trio.lowlevel.open_process`,
  - The ``aclose`` method on `Process` has been deprecated along with
    ``async with process_obj``.
- Now context variables set with `contextvars` are preserved when running functions
  in a worker thread with `trio.to_thread.run_sync`, or when running
  functions from the worker thread in the parent Trio thread with
  `trio.from_thread.run`, and `trio.from_thread.run_sync`.
  This is done by automatically copying the `contextvars` context.
  `trio.lowlevel.spawn_system_task` now also receives an optional ``context`` \ 
argument.

Bugfixes
- Trio now avoids creating cyclic garbage when a `MultiError` is generated and \ 
filtered,
  including invisibly within the cancellation system.  This means errors raised
  through nurseries and cancel scopes should result in less GC latency.
- Trio now deterministically cleans up file descriptors that were opened before
  subprocess creation fails. Previously, they would remain open until the next run of
  the garbage collector.
- Add compatibility with OpenSSL 3.0 on newer Python and PyPy versions by working
  around ``SSLEOFError`` not being raised properly.
- Fix a bug that could cause `Process.wait` to hang on Linux systems using pidfds, if
  another task were to access `Process.returncode` after the process exited but \ 
before
  ``wait`` woke up