Bump version to 2.1.dev0

next master verison is a new feature release; maintenance and 2.0 stable release
happen in release branch

pyproject.toml

 [tool.poetry]
 name = "AlekSIS-Core"
-version = "2.0b1.dev0"
+version = "2.1.dev0"
 packages = [
     { include = "aleksis" }
 ]