Bump version to 2.4.0+dev (#21727)

The release branch has been cut:
https://github.com/python/mypy/tree/release-2.3

Author: JukkaL

mypy/version.py

@@ -8,7 +8,7 @@
 # - Release versions have the form "1.2.3".
 # - Dev versions have the form "1.2.3+dev" (PLUS sign to conform to PEP 440).
 # - Before 1.0 we had the form "0.NNN".
-__version__ = "2.3.0+dev"
+__version__ = "2.4.0+dev"
 base_version = __version__
 
 mypy_dir = os.path.abspath(os.path.dirname(os.path.dirname(__file__)))