Support the latest version of Django

Py3.6 system test would fail with the latest version of Django, need to get it fixed.

============================= test session starts ==============================
platform linux -- Python 3.6.0, pytest-3.2.2, py-1.8.0, pluggy-0.4.0
rootdir: /root/repo, inifile:
plugins: cov-2.5.1
collected 14 items                                                              

tests/system/stats/prometheus/prometheus_stats_test.py .
tests/system/stats/stackdriver/stackdriver_stats_test.py Dropping 1 non-positive bucket boundaries
.Dropping 1 non-positive bucket boundaries
.
tests/system/trace/basic_trace/basic_trace_system_test.py Test simple tracing...
Test simple tracing...
.
tests/system/trace/django/django_system_test.py 95ba383f898c4fc1abf49ccc23f50ad9/a226ee08ca664278;o=1
--2019-03-04 22:55:31--  http://localhost:8000/
Resolving localhost (localhost)... 127.0.0.1, 127.0.0.1, ::1
Connecting to localhost (localhost)|127.0.0.1|:8000... failed: Connection refused.
Connecting to localhost (localhost)|127.0.0.1|:8000... failed: Connection refused.
Connecting to localhost (localhost)|::1|:8000... failed: Cannot assign requested address.
Retrying.

Unhandled exception in thread started by <function check_errors.<locals>.wrapper at 0x7f148153fa60>
Traceback (most recent call last):
  File "/root/repo/.nox/sys-3-6/lib/python3.6/site-packages/django/urls/conf.py", line 17, in include
    urlconf_module, app_name = arg
ValueError: too many values to unpack (expected 2)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/root/repo/.nox/sys-3-6/lib/python3.6/site-packages/django/utils/autoreload.py", line 225, in wrapper
    fn(*args, **kwargs)
  File "/root/repo/.nox/sys-3-6/lib/python3.6/site-packages/django/core/management/commands/runserver.py", line 117, in inner_run
    self.check(display_num_errors=True)
  File "/root/repo/.nox/sys-3-6/lib/python3.6/site-packages/django/core/management/base.py", line 379, in check
    include_deployment_checks=include_deployment_checks,
  File "/root/repo/.nox/sys-3-6/lib/python3.6/site-packages/django/core/management/base.py", line 366, in _run_checks
    return checks.run_checks(**kwargs)
  File "/root/repo/.nox/sys-3-6/lib/python3.6/site-packages/django/core/checks/registry.py", line 71, in run_checks
    new_errors = check(app_configs=app_configs)
  File "/root/repo/.nox/sys-3-6/lib/python3.6/site-packages/django/core/checks/urls.py", line 40, in check_url_namespaces_unique
    all_namespaces = _load_all_namespaces(resolver)
  File "/root/repo/.nox/sys-3-6/lib/python3.6/site-packages/django/core/checks/urls.py", line 57, in _load_all_namespaces
    url_patterns = getattr(resolver, 'url_patterns', [])
  File "/root/repo/.nox/sys-3-6/lib/python3.6/site-packages/django/utils/functional.py", line 37, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/root/repo/.nox/sys-3-6/lib/python3.6/site-packages/django/urls/resolvers.py", line 533, in url_patterns
    patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
  File "/root/repo/.nox/sys-3-6/lib/python3.6/site-packages/django/utils/functional.py", line 37, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/root/repo/.nox/sys-3-6/lib/python3.6/site-packages/django/urls/resolvers.py", line 526, in urlconf_module
    return import_module(self.urlconf_name)
  File "/root/repo/.nox/sys-3-6/lib/python3.6/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 978, in _gcd_import
  File "<frozen importlib._bootstrap>", line 961, in _find_and_load
  File "<frozen importlib._bootstrap>", line 950, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 655, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 678, in exec_module
  File "<frozen importlib._bootstrap>", line 205, in _call_with_frames_removed
  File "/root/repo/tests/system/trace/django/app/urls.py", line 37, in <module>
    url(r'^admin/', include(admin.site.urls)),
  File "/root/repo/.nox/sys-3-6/lib/python3.6/site-packages/django/urls/conf.py", line 27, in include
    'provide the namespace argument to include() instead.' % len(arg)
django.core.exceptions.ImproperlyConfigured: Passing a 3-tuple to include() is not supported. Pass a 2-tuple containing the list of patterns and app_name, and provide the namespace argument to include() instead.

[reyang]

@odeke-em FYI, @zoidbergwill is working on supporting Django 2.x in #358, which is using connection.execute_wrappers.

[timgraham]

It looks to me like existing versions of OpencensusMiddleware should work with fine with Django 2.x. The test failures only required changes in tests (#694). https://github.com/census-instrumentation/opencensus-python/pull/697 removes the install_requires restriction for Django 2.x.

As far as I can tell, #358 is a new feature and has nothing to do with getting existing functionality working with Django 2.x.