Default to gztar for sdists on all platforms. Ref #748.

CHANGES.rst

@@ -2,6 +2,13 @@
 CHANGES
 =======
 
+v26.0.0
+-------
+
+* #748: By default, sdists are now produced in gzipped tarfile
+  format by default on all platforms, adding forward compatibility
+  for the same behavior in Python 3.6 (See Python #27819).
+
 v25.4.0
 -------
 

setuptools/command/sdist.py

@@ -66,6 +66,17 @@ class sdist(orig.sdist):
             if data not in dist_files:
                 dist_files.append(data)
 
+    def initialize_options(self):
+        orig.sdist.initialize_options(self)
+
+        self._default_to_gztar()
+
+    def _default_to_gztar(self):
+        # only needed on Python prior to 3.6.
+        if sys.version_info >= (3, 6, 0, 'beta', 1):
+            return
+        self.formats = ['gztar']
+
     def make_distribution(self):
         """
         Workaround for #516