Clean up Django 2 path backslashes

In Django 2, routes defines via urls.path are aggresively escaped when converted into regex.

This is a naive fix which unescapes all characters outside capture groups, but in the context of OpenAPI is okay because regular expressions inside paths are not supported anyway.

This issue affects django-rest-framework as well, as outlined in encode/django-rest-framework#5672, encode/django-rest-framework#5675.

docs/changelog.rst

@@ -8,6 +8,8 @@ Changelog
 *********
 
 - **FIX:** fixed a crash caused by having read-only Serializers nested by reference
+- **FIX:** removed erroneous backslashes in paths when routes are generated using Django 2
+  `path() <https://docs.djangoproject.com/en/2.0/ref/urls/#django.urls.path>`_
 - **IMPROVEMENT:** updated ``swagger-ui`` to version 3.7.0
 
 *********

docs/conf.py

@@ -168,6 +168,7 @@ nitpick_ignore = [
 
     ('py:class', 'ruamel.yaml.dumper.SafeDumper'),
     ('py:class', 'rest_framework.renderers.BaseRenderer'),
+    ('py:class', 'rest_framework.schemas.generators.EndpointEnumerator'),
     ('py:class', 'rest_framework.views.APIView'),
 
     ('py:class', 'OpenAPICodecYaml'),