mesonlib: Use class syntax for defining MachineChoice

Mypy struggles with the imperative form of Enum declaration, and
upstream doesn't consider it a bug, they recomend using the class form
for enums that are going to be externally exposed.

mesonbuild/mesonlib.py

@@ -308,7 +308,15 @@ class OrderedEnum(Enum):
             return self.value < other.value
         return NotImplemented
 
-MachineChoice = OrderedEnum('MachineChoice', ['BUILD', 'HOST', 'TARGET'])
+class MachineChoice(OrderedEnum):
+
+    """Enum class representing one of the three possible values for binaries,
+    the build, host, and target machines.
+    """
+
+    BUILD = 0
+    HOST = 1
+    TARGET = 2
 
 class PerMachine:
     def __init__(self, build, host, target):