Fix mistaken return type

This return value is not used anywhere (clearly).

Author: WhyNotHugo

vdirsyncer/utils.py

@@ -106,7 +106,7 @@ class can take, and ``required`` is the subset of arguments the class
     return all, required
 
 
-def checkdir(path: str, create: bool = False, mode: int = 0o750) -> bool:
+def checkdir(path: str, create: bool = False, mode: int = 0o750) -> None:
     """Check whether ``path`` is a directory.
 
     :param create: Whether to create the directory (and all parent directories)