fixed mypy error

Author: ckunki

exasol/toolbox/util/git.py

@@ -49,5 +49,5 @@ def create_and_switch_to_branch(branch_name: str):
 
     @staticmethod
     @run_command
-    def toplevel() -> str:
+    def toplevel():
         return ["git", "rev-parse", "--show-toplevel"]