Update version to 0.981

There have changes pushed to the branch since the version was updated
to 0.980. By updating the version we'll get a new set of wheels built.

Author: JukkaL

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__ = "0.980"
+__version__ = "0.981"
 base_version = __version__
 
 mypy_dir = os.path.abspath(os.path.dirname(os.path.dirname(__file__)))