core: chroot: make ChrootType membership check work on python 3.10

You can't check enum membership with __contains__() on Python 3.10, poke
inside the enum instead.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>

pmb/core/chroot.py

@@ -50,7 +50,7 @@ class Chroot:
             "riscv64",
         ]
 
-        if self.__type not in ChrootType:
+        if self.__type not in ChrootType._member_map_.values():
             raise ValueError(f"Invalid chroot type: '{self.__type}'")
 
         # A buildroot suffix must have a name matching one of alpines