Simplify test matrix for old py3 versions.

Makefile

@@ -181,10 +181,10 @@ test-py27: $(PY27)
 	PYTHON=python2.7.14 PATH=$(BUILD_RUNTIMES)/versions/python2.7.14/bin:$(PATH) make develop lint leaktest allbackendtest
 
 test-py34: $(PY34)
-	PYTHON=python3.4.7 PATH=$(BUILD_RUNTIMES)/versions/python3.4.7/bin:$(PATH) make develop allbackendtest
+	PYTHON=python3.4.7 PATH=$(BUILD_RUNTIMES)/versions/python3.4.7/bin:$(PATH) make develop basictest
 
 test-py35: $(PY35)
-	PYTHON=python3.5.4 PATH=$(BUILD_RUNTIMES)/versions/python3.5.4/bin:$(PATH) make develop allbackendtest
+	PYTHON=python3.5.4 PATH=$(BUILD_RUNTIMES)/versions/python3.5.4/bin:$(PATH) make develop basictest
 
 test-py36: $(PY36)
 	PYTHON=python3.6.4 PATH=$(BUILD_RUNTIMES)/versions/python3.6.4/bin:$(PATH) make develop allbackendtest

src/gevent/threading.py

@@ -87,6 +87,10 @@ class _DummyThread(_DummyThread_):
     # For the same reason, instances of this class will cleanup their own entry
     # in ``threading._active``
 
+    # This class also solves a problem forking process with subprocess: after forking,
+    # Thread.__stop is called, which throws an exception when __block doesn't
+    # exist.
+
     # Capture the static things as class vars to save on memory/
     # construction time.
     # In Py2, they're all private; in Py3, they become protected