Improve the setuptools version check (#322)

Handles malformed `setuptools.__version__` such as "41.4.0.post20191022"

Author: primeos

setup.py

@@ -85,7 +85,7 @@ def howto_install_setuptools():
 
     setup, Command = setuptools.setup, setuptools.Command
 
-    observed_version = [int(x) for x in setuptools.__version__.split('.')]
+    observed_version = [int(x) for x in setuptools.__version__.split('.')[:3]]
     required_version = [36, 2, 0]
 
     # NOTE(etingof): require fresh setuptools to build proper wheels