Fix Pytest4.x compatibility errors

There are no longer named marker attributes:
pytest-dev/pytest#891

This results in not expecting param `ssl` and leads to errors like:
```
...
test/test_dugong.py::test_http_proxy[plain-None] PASSED                  [  4%]
test/test_dugong.py::test_http_proxy[plain-8080] PASSED                  [  5%]
test/test_dugong.py::test_connect_proxy[plain-None] PASSED               [  6%]
test/test_dugong.py::test_connect_proxy[plain-8080] PASSED               [  7%]
...
test/test_dugong.py::test_http_proxy[ssl-None] FAILED                    [ 51%]
test/test_dugong.py::test_http_proxy[ssl-8080] FAILED                    [ 52%]
test/test_dugong.py::test_connect_proxy[ssl-None] FAILED                 [ 53%]
test/test_dugong.py::test_connect_proxy[ssl-8080] FAILED                 [ 54%]
...

So, the marker `no_ssl` is just ignored.
Let's use the `get_closest_marker` method instead.

Signed-off-by: Stanislav Levin <slev@altlinux.org>

setup.cfg

@@ -5,3 +5,7 @@ formats=bztar
 source-dir = rst
 build-dir  = doc
 
+[tool:pytest]
+markers =
+    no_ssl
+

test/test_dugong.py

@@ -83,7 +83,7 @@ def pytest_generate_tests(metafunc):
     if not 'http_server' in metafunc.fixturenames:
         return
 
-    if getattr(metafunc.function, 'no_ssl', False):
+    if metafunc.definition.get_closest_marker('no_ssl'):
         params = ('plain',)
     else:
         params = ('plain', 'ssl')