Resend: Avoid installing broken version of svix package

Author: medmunds

CHANGELOG.rst

@@ -56,6 +56,9 @@ Fixes
 * **Postmark:** Fix an error in inbound handling with long email address display
   names that include non-ASCII characters.
 
+* **Resend:** Avoid installing a broken version of the ``svix`` package
+  (which is used to validate Resend webhooks).
+
 * **SendGrid:** Improve handling of non-string values in ``merge_data`` when using
   legacy templates or inline merge fields. To avoid a confusing SendGrid API error
   message, Anymail now converts numeric merge data values to strings, but will raise

pyproject.toml

@@ -78,7 +78,11 @@ mailgun = []
 mailjet = []
 mandrill = []
 postmark = []
-resend = ["svix"]
+resend = [
+    # Svix v1.63.0 actually requires Python 3.10, but claims otherwise.
+    "svix<1.63.0; python_version < '3.10'",
+    "svix; python_version >= '3.10'"
+]
 sendgrid = []
 sendinblue = []
 sparkpost = []