Fix invalid escape sequence deprecation warnings

When running tests with warnings enabled, appear as:

  DeprecationWarning: invalid escape sequence \d

Starting with Python 3.6, invalid escape sequences are deprecated. In a
future Python versions they will be a syntax error. For more details, see:

https://docs.python.org/3/whatsnew/3.6.html#deprecated-python-behavior

> A backslash-character pair that is not a valid escape sequence now
> generates a DeprecationWarning. Although this will eventually become a
> SyntaxError, that will not be for several Python releases.

Author: jdufresne

tests/test_schemas.py

@@ -313,9 +313,9 @@ def get(self, *args, **kwargs):
 class TestSchemaGenerator(TestCase):
     def setUp(self):
         self.patterns = [
-            url('^example/?$', ExampleListView.as_view()),
-            url('^example/(?P<pk>\d+)/?$', ExampleDetailView.as_view()),
-            url('^example/(?P<pk>\d+)/sub/?$', ExampleDetailView.as_view()),
+            url(r'^example/?$', ExampleListView.as_view()),
+            url(r'^example/(?P<pk>\d+)/?$', ExampleDetailView.as_view()),
+            url(r'^example/(?P<pk>\d+)/sub/?$', ExampleDetailView.as_view()),
         ]
 
     def test_schema_for_regular_views(self):
@@ -365,9 +365,9 @@ def test_schema_for_regular_views(self):
 class TestSchemaGeneratorNotAtRoot(TestCase):
     def setUp(self):
         self.patterns = [
-            url('^api/v1/example/?$', ExampleListView.as_view()),
-            url('^api/v1/example/(?P<pk>\d+)/?$', ExampleDetailView.as_view()),
-            url('^api/v1/example/(?P<pk>\d+)/sub/?$', ExampleDetailView.as_view()),
+            url(r'^api/v1/example/?$', ExampleListView.as_view()),
+            url(r'^api/v1/example/(?P<pk>\d+)/?$', ExampleDetailView.as_view()),
+            url(r'^api/v1/example/(?P<pk>\d+)/sub/?$', ExampleDetailView.as_view()),
         ]
 
     def test_schema_for_regular_views(self):