__init__.py: Update version to 3.1.0.dev

Following https://packaging.python.org/en/latest/discussions/versioning/

mesa/__init__.py

@@ -22,7 +22,7 @@
 ]
 
 __title__ = "mesa"
-__version__ = "3.0.1"
+__version__ = "3.1.0.dev"
 __license__ = "Apache 2.0"
 _this_year = datetime.datetime.now(tz=datetime.UTC).date().year
 __copyright__ = f"Copyright {_this_year} Project Mesa Team"