tests: Ignore tests using 'collect_ignore'

Per the pytest docs [1], this is the preferred way to ignore tests. This
necessitates removing the 'test-async' target as it no longer makes any
sense.

[1] https://docs.pytest.org/en/latest/example/pythoncollection.html

Signed-off-by: Stephen Finucane <stephen@that.guru>

tests/conftest.py

@@ -8,12 +8,17 @@
 """
 
 import os
+import sys
 
 import pytest
 from sphinx.testing.path import path
 
 pytest_plugins = 'sphinx.testing.fixtures'
 
+# Disable Python version-specific
+if sys.version_info < (3, 5):
+    collect_ignore = ['py35']
+
 
 @pytest.fixture(scope='session')
 def rootdir():