- Add @pytest_trio.trio_fixture for explicitly marking a fixture as
  being a trio fixture
- Make the nursery fixture a @trio_fixture
- Refactor Trio fixture classes into one class
- Check for trio marker instead of trio keyword (fixes python-triogh-43)
  - This also raises the minimum pytest version to 3.6
- Raise an error if a Trio fixture is used with a non-function
  scope (fixes python-triogh-18)
- Raise an error if a Trio fixture is used with a non-Trio test

I think this also closes python-triogh-10's discussion, though we still need to
convince pytest-asyncio to fix their side of things.

Every nursery is called 'nursery'. This is less generic, and
emphasizes that this nursery is special, because its lifetime is
linked to the test.

Kept 'nursery' around for now as a deprecated alias.

The main benefit of this is that it lets us catch more cases where
these fixtures are accidentally misused.

Closes python-triogh-18
Closes python-triogh-51

See python-trio#55 for discussion.

Main points:

* Fixture setup/teardown is now performed concurrently whenever
  possible.

* Each fixture is now run in an isolated task, and it's now safe to
  use nurseries inside fixtures (fixes python-triogh-55).

* New magical fixture 'fixture_nursery' which gives a nursery that's
  cancelled immediately after *the requesting fixture finishes
  teardown*. If requested directly by the test itself, gives a nursery
  which is cancelled after the test finishes, before other fixtures
  are torn down.

At this point this:

@pytest.fixture
async fix(nursery):
    ...

is just a shorthand for this (they should be exactly equivalent):

@pytest.fixture
async fix():
    async with trio.open_nursery() as nursery:
        try:
            ...
        finally:
            nursery.cancel_scope.cancel()

And the latter is the obvious thing you would write by hand, there's
really no other way to do it. And since it no longer has special
subtle semantics, I'm no longer bothered by using the short name
'nursery'. Plus the longer I looked at 'fixture_nursery' the more
annoying the name seemed. Plus I made that last minute change so you
can use the magic nursery inside tests too, not just fixtures, so the
name didn't even make much sense.

This is sort of a gross hack, but it's simple and works. If it causes
problems, then this comment discusses some other options:
  python-trio#50 (comment)

Also added tests for using trio-asyncio in fixtures (which was the
motivating use case for sharing contextvars).

When importing trio-asyncio v0.8.0 on py35, we get a crash at import
time:

  .../python3.5/site-packages/trio_asyncio/loop.py:171: in <module>
      _orig_run_get = _aio_event._get_running_loop
  E   AttributeError: module 'asyncio.events' has no attribute '_get_running_loop'

See: https://travis-ci.org/python-trio/pytest-trio/jobs/419092404

So let's skip testing this for now...