bpo-22831: Use "with" to avoid possible fd leaks in distutils. (GH-10921)

diff --git a/Lib/distutils/command/sdist.py b/Lib/distutils/command/sdist.py
index 52eaa15..b4996fc 100644
--- a/Lib/distutils/command/sdist.py
+++ b/Lib/distutils/command/sdist.py
@@ -407,14 +407,13 @@
         distribution.
         """
         log.info("reading manifest file '%s'", self.manifest)
-        manifest = open(self.manifest)
-        for line in manifest:
-            # ignore comments and blank lines
-            line = line.strip()
-            if line.startswith('#') or not line:
-                continue
-            self.filelist.append(line)
-        manifest.close()
+        with open(self.manifest) as manifest:
+            for line in manifest:
+                # ignore comments and blank lines
+                line = line.strip()
+                if line.startswith('#') or not line:
+                    continue
+                self.filelist.append(line)
 
     def make_release_tree(self, base_dir, files):
         """Create the directory tree that will become the source