Update 'bdist' format addition to assume a single 'format_commands' as a dictionary, but fall back to the dual dict/list model for compatibility with stdlib.

Author: jaraco

setuptools/command/__init__.py

@@ -2,7 +2,11 @@
 import sys
 
 if 'egg' not in bdist.format_commands:
-    bdist.format_command['egg'] = ('bdist_egg', "Python .egg file")
-    bdist.format_commands.append('egg')
+    try:
+        bdist.format_commands['egg'] = ('bdist_egg', "Python .egg file")
+    except TypeError:
+        # For backward compatibility with older distutils (stdlib)
+        bdist.format_command['egg'] = ('bdist_egg', "Python .egg file")
+        bdist.format_commands.append('egg')
 
 del bdist, sys