This will stop filters without stopping the entire runner.

This should make no difference to current code, but may help defend
against future breakages.

Closes #10

Closes #11

Closes #9

… but use ~=3.1 for typing, as there are a few typing-related issues in
the 3.0 releases.

The regex could, in particular, match exception messages in doctest
strings in docstrings such as "ValueError: ..."

The test_abort_close runner test fails as the abort->close flow takes
a detour through abort->restart->close in the current implementation.

These two are not valid returns from filters (although Skip is valid as
a message passed through channels from a filter as a message response).

A warning will be issued in the following situation:

```
>>> def filter(session: Session):
...   async with session.body as body_iter:
...     await session.body.write(b"spam")

>>> test_runner(filter)
UserWarning: it looks as if BodyAccessor.write() was called on an
instance from within it's own async context
  await session.body.write(b"spam")
```

Moved the boolean 'skip' attribute from `Session` to `BodyAccessor` as
it seems a more logical place for it. Although users *shouldn't* modify
the attribute, if they do it *may* work and this is a better place for
them to find it.

This allows synchronisation of any early response, before broadcast
messages are awaited.

Closes #8

Closes #7