Postmark inbound: improve inbound parsing

- Support Postmark's RawEmail option;
  recommend it in docs
- Handle Bcc when provided by Postmark
- Obtain `envelope_sender` from Return-Path info
  Postmark now adds, rather than parsing Received-SPF

Related:
- Add `AnymailInboundMessage.bcc` convenience prop
- Test against full Postmark "check" inbound payloads
  (which don't match their docs or real inbound payloads)
- Don't warn about receiving "check" payload

CHANGELOG.rst

@@ -59,6 +59,19 @@ Breaking changes
 
 * Require urllib3 1.25 or later (released 2019-04-29).
 
+Features
+~~~~~~~~
+
+* **Postmark inbound:**
+
+  * Handle Postmark's "Include raw email content in JSON payload"
+    inbound option. Enabling this setting is recommended to get
+    the most accurate representation of any received email.
+  * Obtain ``envelope_sender`` from *Return-Path* Postmark now provides.
+    (Replaces potentially faulty *Received-SPF* header parsing.)
+  * Add *Bcc* header to inbound message if provided. Postmark adds bcc
+    when the delivered-to address does not appear in the *To* header.
+
 Other
 ~~~~~