Mailgun: disable non-ASCII attachment filename workaround when not needed

urllib3 v1.25 fixes non-ASCII filenames in multipart form data to be
RFC 5758 compliant by default, so our earlier workaround is no longer
needed. Disable the workaround if we detect that Requests is using a
fixed version of urllib3.

Closes #157

anymail/backends/mailgun.py

@@ -11,6 +11,21 @@ from ..utils import get_anymail_setting, rfc2822date
 from .base_requests import AnymailRequestsBackend, RequestsPayload
 
 
+# Feature-detect whether requests (urllib3) correctly uses RFC 7578 encoding for non-
+# ASCII filenames in Content-Disposition headers. (This was fixed in urllib3 v1.25.)
+# See MailgunPayload.get_request_params for info (and a workaround on older versions).
+# (Note: when this workaround is removed, please also remove the "old_urllib3" tox envs.)
+def is_requests_rfc_5758_compliant():
+    request = Request(method='POST', url='https://www.example.com',
+                      files=[('attachment', (u'\N{NOT SIGN}.txt', 'test', 'text/plain'))])
+    prepared = request.prepare()
+    form_data = prepared.body  # bytes
+    return b'filename*=' not in form_data
+
+
+REQUESTS_IS_RFC_7578_COMPLIANT = is_requests_rfc_5758_compliant()
+
+
 class EmailBackend(AnymailRequestsBackend):
     """
     Mailgun API Email Backend
@@ -94,7 +109,7 @@ class MailgunPayload(RequestsPayload):
         non_ascii_filenames = [filename
                                for (field, (filename, content, mimetype)) in params["files"]
                                if filename is not None and not isascii(filename)]
-        if non_ascii_filenames:
+        if non_ascii_filenames and not REQUESTS_IS_RFC_7578_COMPLIANT:
             # Workaround https://github.com/requests/requests/issues/4652:
             # Mailgun expects RFC 7578 compliant multipart/form-data, and is confused
             # by Requests/urllib3's improper use of RFC 2231 encoded filename parameters