Upgrade to pants 2.12.0 (#101)

Author: asherf

pants.toml

@@ -2,7 +2,7 @@
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
 [GLOBAL]
-pants_version = "2.11.0rc5"
+pants_version = "2.12.0"
 backend_packages.add = [
   "pants.backend.python",
   "pants.backend.python.lint.docformatter",