test: Ignore 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 we ignore this test for
that Python version.

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.

That's why we ignore this test for CPython 3.7.

[1] python-trio/pytest-trio#126
[2] python/cpython#9201

Signed-off-by: Michael Seifert <[email protected]>

tests/test_subprocess.py

@@ -15,6 +15,18 @@ def event_loop():
         loop.close()
 
 
+@pytest.mark.skipif(
+    sys.version_info < (3, 8),
+    reason="""
+        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 we ignore this test for
+        that Python version.
+
+        [1] https://github.com/python-trio/pytest-trio/issues/126
+    """,
+)
 @pytest.mark.asyncio
 async def test_subprocess(event_loop):
     """Starting a subprocess should be possible."""