Python 3.8 (prep): Import Mapping etc. from collections.abc

Python 3.3 moved various collections abstract base classes from
`collections` to `collections.abc`, but also kept them available in
`collections` for compatibility with Python 2. Python 3.8 will allow
importing only from `collections.abc`.

(`collections.abc` hasn't yet been added to six.moves; see
https://github.com/benjaminp/six/issues/155.)

anymail/backends/sendgrid.py

@@ -1,14 +1,13 @@
 import uuid
-from collections import Mapping
-from email.utils import quote as rfc822_quote
 import warnings
+from email.utils import quote as rfc822_quote
 
 from requests.structures import CaseInsensitiveDict
 
 from .base_requests import AnymailRequestsBackend, RequestsPayload
 from ..exceptions import AnymailConfigurationError, AnymailRequestsAPIError, AnymailWarning
 from ..message import AnymailRecipientStatus
-from ..utils import BASIC_NUMERIC_TYPES, get_anymail_setting, timestamp, update_deep
+from ..utils import BASIC_NUMERIC_TYPES, Mapping, get_anymail_setting, timestamp, update_deep
 
 
 class EmailBackend(AnymailRequestsBackend):

anymail/utils.py

@@ -1,7 +1,6 @@
 import base64
 import mimetypes
 from base64 import b64encode
-from collections import Mapping, MutableMapping
 from datetime import datetime
 from email.mime.base import MIMEBase
 from email.utils import formatdate, getaddresses, unquote
@@ -15,6 +14,11 @@ from django.utils.functional import Promise
 from django.utils.timezone import utc, get_fixed_timezone
 from six.moves.urllib.parse import urlsplit, urlunsplit
 
+try:
+    from collections.abc import Mapping, MutableMapping  # Python 3.3+
+except ImportError:
+    from collections import Mapping, MutableMapping
+
 from .exceptions import AnymailConfigurationError, AnymailInvalidAddress