Use string values for django-cache keys #230 (#242)

* Use string values for django-chache keys.

When django cache backend tests the key provided by django_celery_results, It wants to have str type value. 
byte values result in the following exception being raised:

  File "/opt/exaboard/eXaboard/lib/python3.6/site-packages/django/core/cache/backends/base.py", line 287, in memcache_key_warnings
    if ord(char) < 33 or ord(char) == 127:
TypeError: ord() expected string of length 1, but int found

* Remove old super call argument

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Add test that saves key value in byte format

* Syntax fix for test_convert_key_from_byte_to_str

* Fixes based on flake8 failures

* Fix t/unit/backends/test_cache.py:64:50: W291 trailing whitespace

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

django_celery_results/backends/cache.py

@@ -3,6 +3,7 @@
 from celery.backends.base import KeyValueStoreBackend
 from django.core.cache import cache as default_cache
 from django.core.cache import caches
+from kombu.utils.encoding import bytes_to_str
 
 
 class CacheBackend(KeyValueStoreBackend):
@@ -15,12 +16,15 @@ def __init__(self, *args, **kwargs):
         self.serializer = 'pickle'
 
     def get(self, key):
+        key = bytes_to_str(key)
         return self.cache_backend.get(key)
 
     def set(self, key, value):
+        key = bytes_to_str(key)
         self.cache_backend.set(key, value, self.expires)
 
     def delete(self, key):
+        key = bytes_to_str(key)
         self.cache_backend.delete(key)
 
     def encode(self, data):

t/unit/backends/test_cache.py

@@ -4,6 +4,7 @@
 import pytest
 from billiard.einfo import ExceptionInfo
 from celery import result, states, uuid
+from kombu.utils.encoding import bytes_to_str
 
 from django_celery_results.backends.cache import CacheBackend
 
@@ -59,6 +60,18 @@ def test_is_pickled(self):
         assert rindb.get('foo') == 'baz'
         assert rindb.get('bar').data == 12345
 
+    def test_convert_key_from_byte_to_str(self):
+        """ Tests that key in byte form passed into cache
+            are succesfully completed """
+        tid = bytes_to_str(uuid())
+
+        assert self.b.get_status(tid) == states.PENDING
+        assert self.b.get_result(tid) is None
+
+        self.b.mark_as_done(tid, 42)
+        assert self.b.get_status(tid) == states.SUCCESS
+        assert self.b.get_result(tid) == 42
+
     def test_mark_as_failure(self):
         einfo = None
         tid3 = uuid()