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

tox.ini

@@ -15,6 +15,9 @@ envlist =
     djangoMaster-py{36,37}-all
     # ... then partial installation (limit extras):
     django22-py37-{none,amazon_ses,sparkpost}
+    # ... then older versions of some dependencies:
+    django111-py27-all-old_urllib3
+    django22-py37-all-old_urllib3
 
 [testenv]
 deps =
@@ -23,6 +26,7 @@ deps =
     django21: django~=2.1.0
     django22: django~=2.2.0
     djangoMaster: https://github.com/django/django/tarball/master
+    old_urllib3: urllib3<1.25
     # testing dependencies (duplicates setup.py tests_require, less optional extras):
     mock
 extras =