MNT: Mark incompatible with Python 3.9

Author: effigies

Changelog

@@ -28,6 +28,9 @@ References like "pr/298" refer to github pull request numbers.
 2.5.2 (To Be Determined)
 ========================
 
+This and all future releases in 2.5.x series will be incompatible with Python
+3.9.
+
 Bug fixes
 ---------
 * Change strings with invalid escapes to raw strings (pr/827) (EL, reviewed

setup.cfg

@@ -29,7 +29,7 @@ provides =
     nisext
 
 [options]
-python_requires = >=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*
+python_requires = >=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,<3.9dev
 install_requires =
     numpy >=1.8
     six >=1.3