Allow idna 3.x to be installed on Python 3.x

Co-authored-by: Mickaël Schoentgen <contact@tiger-222.fr>
Co-authored-by: Seth Michael Larson <sethmichaellarson@gmail.com>

setup.py

@@ -43,7 +43,8 @@ def run_tests(self):
 requires = [
     'charset_normalizer~=2.0.0; python_version >= "3"',
     'chardet>=3.0.2,<5; python_version < "3"',
-    'idna>=2.5,<3',
+    'idna>=2.5,<3; python_version < "3"',
+    'idna>=2.5,<4; python_version >= "3"',
     'urllib3>=1.21.1,<1.27',
     'certifi>=2017.4.17'