Improve testing (#415)

These changes:

 - Fix the lint and the djmaster jobs
 - Fix compatibility with upcoming Django 3.0
 - Replace jobs with matrix in .travis.yml
 - Add a test job for Python 3.8
 - Allow running tests on any branch

src/drf_yasg/inspectors/__init__.py

@@ -5,7 +5,7 @@ from .base import (
 from .field import (
     CamelCaseJSONFilter, ChoiceFieldInspector, DictFieldInspector, FileFieldInspector, HiddenFieldInspector,
     InlineSerializerInspector, JSONFieldInspector, RecursiveFieldInspector, ReferencingSerializerInspector,
-     RelatedFieldInspector, SerializerMethodFieldInspector, SimpleFieldInspector, StringDefaultFieldInspector
+    RelatedFieldInspector, SerializerMethodFieldInspector, SimpleFieldInspector, StringDefaultFieldInspector
 )
 from .query import CoreAPICompatInspector, DjangoRestResponsePagination
 from .view import SwaggerAutoSchema

src/drf_yasg/renderers.py

@@ -2,7 +2,7 @@ import six
 
 from django.shortcuts import resolve_url
 from django.template.loader import render_to_string
-from django.utils.encoding import force_text
+from django.utils.encoding import force_str
 from django.utils.functional import Promise
 from rest_framework.renderers import BaseRenderer, JSONRenderer, TemplateHTMLRenderer
 from rest_framework.utils import encoders, json
@@ -124,7 +124,7 @@ class SwaggerUIRenderer(_UIRenderer):
         swagger_ui_settings = self.get_swagger_ui_settings()
 
         request = renderer_context.get('request', None)
-        oauth_redirect_url = force_text(swagger_ui_settings.get('oauth2RedirectUrl', ''))
+        oauth_redirect_url = force_str(swagger_ui_settings.get('oauth2RedirectUrl', ''))
         if request and oauth_redirect_url:
             swagger_ui_settings['oauth2RedirectUrl'] = request.build_absolute_uri(oauth_redirect_url)
 

src/drf_yasg/utils.py

@@ -5,7 +5,7 @@ from collections import OrderedDict
 from decimal import Decimal
 
 from django.db import models
-from django.utils.encoding import force_text
+from django.utils.encoding import force_str
 from rest_framework import serializers, status
 from rest_framework.mixins import DestroyModelMixin, ListModelMixin, RetrieveModelMixin, UpdateModelMixin
 from rest_framework.parsers import FileUploadParser
@@ -434,7 +434,7 @@ def force_real_str(s, encoding='utf-8', strings_only=False, errors='strict'):
     Fix for https://github.com/axnsan12/drf-yasg/issues/159
     """
     if s is not None:
-        s = force_text(s, encoding, strings_only, errors)
+        s = force_str(s, encoding, strings_only, errors)
         if type(s) != str:
             s = '' + s
 

src/drf_yasg/views.py

@@ -1,8 +1,7 @@
 import warnings
-from functools import wraps
+from functools import WRAPPER_ASSIGNMENTS, wraps
 
 from django.utils.cache import add_never_cache_headers
-from django.utils.decorators import available_attrs
 from django.views.decorators.cache import cache_page
 from django.views.decorators.vary import vary_on_headers
 from rest_framework import exceptions
@@ -30,7 +29,7 @@ def deferred_never_cache(view_func):
     never be cached.
     """
 
-    @wraps(view_func, assigned=available_attrs(view_func))
+    @wraps(view_func, assigned=WRAPPER_ASSIGNMENTS)
     def _wrapped_view_func(request, *args, **kwargs):
         response = view_func(request, *args, **kwargs)