Fix incorrect mtime comparison in distutils (#11933).

This is a regression introduced in 9211a5d7d0b4, when uses of ST_MTIME
constants were changed to uses of st_mtime attributes.  As diagnosed in
the bug report, this change is not merely stylistic: st_mtime is a
float but ST_MTIME’s resolution is rounded to the seconds, so there was
a mismatch between the values seen by file_util and dep_util which
caused an sdist to be unnecessarily created a second time on an ext4
filesystem.

This patch has been tested by John S. Gruber, who reported the bug.
As this is a simple code revert, I think it’s okay to commit without a
unit test.

dep_util.py

@@ -7,6 +7,7 @@ timestamp dependency analysis."""
 __revision__ = "$Id$"
 
 import os
+from stat import ST_MTIME
 from distutils.errors import DistutilsFileError
 
 def newer(source, target):
@@ -27,7 +28,7 @@ def newer(source, target):
     if not os.path.exists(target):
         return True
 
-    return os.stat(source).st_mtime > os.stat(target).st_mtime
+    return os.stat(source)[ST_MTIME] > os.stat(target)[ST_MTIME]
 
 def newer_pairwise(sources, targets):
     """Walk two filename lists in parallel, testing if each source is newer
@@ -71,7 +72,7 @@ def newer_group(sources, target, missing='error'):
     # is more recent than 'target', then 'target' is out-of-date and
     # we can immediately return true.  If we fall through to the end
     # of the loop, then 'target' is up-to-date and we return false.
-    target_mtime = os.stat(target).st_mtime
+    target_mtime = os.stat(target)[ST_MTIME]
 
     for source in sources:
         if not os.path.exists(source):
@@ -82,7 +83,7 @@ def newer_group(sources, target, missing='error'):
             elif missing == 'newer':    # missing source means target is
                 return True             #  out-of-date
 
-        if os.stat(source).st_mtime > target_mtime:
+        if os.stat(source)[ST_MTIME] > target_mtime:
             return True
 
     return False