Port eventlet.green.OpenSSL to Python 3

* Fix import syntax for Python 3: use relative imports
* Add unit test

eventlet/green/OpenSSL/__init__.py

@@ -1,5 +1,5 @@
-import rand
+from . import rand
-import crypto
+from . import crypto
-import SSL
+from . import SSL
-import tsafe
+from . import tsafe
-from version import __version__
+from .version import __version__

tests/openssl_test.py

@@ -0,0 +1,17 @@
+import tests
+
+
+def test_import():
+    # https://github.com/eventlet/eventlet/issues/238
+    # Ensure that it's possible to import eventlet.green.OpenSSL.
+    # Most basic test to check Python 3 compatibility.
+    try:
+        import OpenSSL
+    except ImportError:
+        raise tests.SkipTest('need pyopenssl')
+
+    import eventlet.green.OpenSSL.SSL
+    import eventlet.green.OpenSSL.crypto
+    import eventlet.green.OpenSSL.rand
+    import eventlet.green.OpenSSL.tsafe
+    import eventlet.green.OpenSSL.version