Update version to remove "+dev" for releasing 0.991

Author: svalentin

mypy/version.py

@@ -8,7 +8,7 @@
 # - Release versions have the form "0.NNN".
 # - Dev versions have the form "0.NNN+dev" (PLUS sign to conform to PEP 440).
 # - For 1.0 we'll switch back to 1.2.3 form.
-__version__ = "0.991+dev"
+__version__ = "0.991"
 base_version = __version__
 
 mypy_dir = os.path.abspath(os.path.dirname(os.path.dirname(__file__)))