Do not try to initialize async fixtures without explicit asyncio mark in strict mode

[seifertm]

This fixes a bug that breaks compatibility with pytest_trio (see #298).

The PR:

• Adds pytest-trio to the test dependencies.

• Removes test with obsolete "forbid_global_loop".

  forbid_global_loop was an option to pytest.mark.asyncio which was removed in v0.6.0. The two subprocess tests are otherwise identical. Therefore, one of the tests was removed along with the obsolete option.

• Fixes a bug that causes a package-scoped event_loop fixture to leak into other tests.

  The expected behaviour is that the event_loop fixture defined in tests/sessionloop/conftest.py is torn down when all tests in tests/sessionloop are complete. Running the tests with the pytest option --setup-show pointed out that the fixture is torn down at the end of the test session, instead. This is an unintended side effect of the sessionloop test which may affect other tests in the test suite.

  Reducing the fixture scope from "package" to "module" results in the expected behaviour. The module was renamed to reflect the fact that the tests do not use a session scope.

• Ignores subprocess tests when running on CPython 3.7.

  When run with Python 3.7, asyncio.subprocess.create_subprocess_exec seems to be affected by an issue that prevents correct cleanup. Tests using pytest-trio will report that signal handling is already performed by another library and fail. [1] This is possibly a bug in CPython 3.7, so I suggest we ignore this test for that Python version.

  I've been shaving this yak for some time and this is my theory:
  CPython 3.7 uses asyncio.streams.StreamReader and asyncio.streams.StreamWriter to implement asyncio.streams.StreamReaderProtocol and asyncio.subprocess.SubprocessStreamProtocol. StreamReaderProtocol contained cyclic references between the reader and the protocol, which prevented garbage collection. While StreamReaderProtocol received a patch [2], SubprocessStreamProtocol, which is used by create_subprocess_exec, possibly has the same problem, but was not patched as part of CPython 3.7.

  [1] pytest-trio exits with TrioInternalError on Python 3.7 when pytest-asyncio is installed python-trio/pytest-trio#126
  [2] bpo-34638: Store a weak reference to stream reader to break strong references loop python/cpython#9201

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.09%. Comparing base (82d212b) to head (b194dae).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #307      +/-   ##
==========================================
+ Coverage   92.67%   93.09%   +0.41%     
==========================================
  Files           3        3              
  Lines         273      275       +2     
  Branches       40       41       +1     
==========================================
+ Hits          253      256       +3     
  Misses         12       12              
+ Partials        8        7       -1     

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