Amazon SES: confirm webhook subscriptions in SNS topic's own region (#236)

Allow inbound and tracking webhooks using SNS topics from any AWS region.
The topic subscription must be confirmed in the topic's own region (not
the boto3 default), determined by examing the topic's ARN.

Fixes #235

CHANGELOG.rst

@@ -33,6 +33,11 @@ vNext
 Fixes
 ~~~~~
 
+* **Amazon SES:** Support receiving and tracking mail in non-default (or multiple)
+  AWS regions. Anymail now always confirms an SNS subscription in the region where
+  the SNS topic exists, which may be different from the boto3 default. (Thanks to
+  `@mark-mishyn`_ for reporting this.)
+
 * **Postmark:** Fix two different errors when sending with a template but no merge
   data. (Thanks to `@kareemcoding`_ and `@Tobeyforce`_ for reporting them.)
 
@@ -1230,6 +1235,7 @@ Features
 .. _@kika115: https://github.com/kika115
 .. _@Lekensteyn: https://github.com/Lekensteyn
 .. _@lewistaylor: https://github.com/lewistaylor
+.. _@mark-mishyn: https://github.com/mark-mishyn
 .. _@mbk-ok: https://github.com/mbk-ok
 .. _@mwheels: https://github.com/mwheels
 .. _@nuschk: https://github.com/nuschk