Move MISSING to the compat

platformio/project/config.py

@@ -21,7 +21,7 @@ import re
 import click
 
 from platformio import fs
-from platformio.compat import string_types
+from platformio.compat import MISSING, string_types
 from platformio.project import exception
 from platformio.project.options import ProjectOptions
 
@@ -38,9 +38,6 @@ CONFIG_HEADER = """
 """
 
 
-MISSING = object()
-
-
 class ProjectConfigBase(object):
 
     INLINE_COMMENT_RE = re.compile(r"\s+;.*$")