[1.7.x] Fixed #22315 -- str/bytes mismatch in staticfiles

Previously, `ManifestFilesMixin.read_manifest` failed in Python 3
because `json.loads` accepts `str` not `bytes`.

Backport of 86dcac4 from master

django/contrib/staticfiles/storage.py

@@ -292,7 +292,7 @@ def __init__(self, *args, **kwargs):
     def read_manifest(self):
         try:
             with self.open(self.manifest_name) as manifest:
-                return manifest.read()
+                return manifest.read().decode('utf-8')
         except IOError:
             return None
 
@@ -319,7 +319,8 @@ def post_process(self, *args, **kwargs):
         payload = {'paths': self.hashed_files, 'version': self.manifest_version}
         if self.exists(self.manifest_name):
             self.delete(self.manifest_name)
-        self._save(self.manifest_name, ContentFile(json.dumps(payload)))
+        contents = json.dumps(payload).encode('utf-8')
+        self._save(self.manifest_name, ContentFile(contents))
 
 
 class _MappingCache(object):

tests/staticfiles_tests/tests.py

@@ -662,6 +662,11 @@ def test_loaded_cache(self):
                       storage.staticfiles_storage.manifest_version,
                       force_text(manifest_content))
 
+    def test_parse_cache(self):
+        hashed_files = storage.staticfiles_storage.hashed_files
+        manifest = storage.staticfiles_storage.load_manifest()
+        self.assertEqual(hashed_files, manifest)
+
 
 # we set DEBUG to False here since the template tag wouldn't work otherwise
 @override_settings(**dict(