Remove redundant usage of collections(.abc).Mapping

The json.loads method returns a normal python dict without any hook
defined, and type check using the Mapping class is redundant.

In Python 3.8
>>> import json
>>> type(json.loads('{"foo": {"var": "baz"}}'))
<class 'dict'>

In Python 2.7
>>> import json
>>> type(json.loads('{"foo": {"var": "baz"}}'))
<class 'dict'>

With this change we don't have to handle difference between Python 2
and Python 3 and later about import path we should use to import that
abstract class

Closes-Bug: #1936667
Change-Id: I9232311784d1feff2d669455dafde17ed9f751ad

swift/common/middleware/crypto/crypto_utils.py

@@ -14,7 +14,6 @@
 # limitations under the License.
 import base64
 import binascii
-import collections
 import json
 import os
 
@@ -260,7 +259,7 @@ def load_crypto_meta(value, b64decode=True):
         if not isinstance(value, six.string_types):
             raise ValueError('crypto meta not a string')
         val = json.loads(urlparse.unquote_plus(value))
-        if not isinstance(val, collections.Mapping):
+        if not isinstance(val, dict):
             raise ValueError('crypto meta not a Mapping')
         return b64_decode_meta(val)
     except (KeyError, ValueError, TypeError) as err: