ntfy: Handle non-ASCII characters in message

In order to set the message for a notification with an attachment, the
text must be specified in the `Message` request header.  Unfortunately,
HTTP header values are limited to the Latin-1 character set, so Unicode
characters cannot be included.  As of *ntfy* 2.4.0, however, the server
can decode base64-encoded headers using the RFC 2047 scheme.

To maintain compatibility with older *ntfy* servers, the `ntfy` function
will only encode message contents this way if the string cannoto be
encoded as ASCII.

xactfetch.py

@@ -1,3 +1,4 @@
+import base64
 import copy
 import datetime
 import json
@@ -51,7 +52,13 @@ def ntfy(
         if filename:
             headers['Filename'] = filename
         if message:
-            headers['Message'] = message.replace('\n', '\\n')
+            try:
+                message.encode("ascii")
+            except UnicodeEncodeError:
+                message = rfc2047_base64encode(message)
+            else:
+                message = message.replace('\n', '\\n')
+            headers['Message'] = message
         r = requests.put(
             url,
             headers=headers,
@@ -117,6 +124,13 @@ def rbw_code(
     return p.stdout.rstrip('\n')
 
 
+def rfc2047_base64encode(
+    message: str,
+) -> str:
+    encoded = base64.b64encode(message.encode("utf-8")).decode("ascii")
+    return f"=?UTF-8?B?{encoded}?="
+
+
 def firefly_import(csv: Path, config: dict[str, Any], token: str) -> None:
     log.debug('Importing transactions from %s to Firefly III', csv)
     env = {