Avoid DeprecationWarning: 'U' mode is deprecated

Python 3.7 began warning about open's 'U' mode. Universal newline mode (newline=None) is default in Python 3 so avoid specifying it on Python 3.

(cherry picked from commit 978a6af9e78db44b9db25cb08c7f9d98c24bbc03)

src/zc/buildout/easy_install.py

@@ -1586,6 +1586,11 @@ def _pyscript(path, dest, rsetup, initialization=''):
     generated.append(dest)
     return generated
 
+if sys.version_info[0] < 3:
+    universal_newline_option = ", 'U'"
+else:
+    universal_newline_option = ''
+
 py_script_template = script_header + '''\
 
 %(relative_paths_setup)s
@@ -1606,19 +1611,19 @@ if len(sys.argv) > 1:
             sys.argv[1:] = _args
             _args = []
             __import__("runpy").run_module(
-                 _val, {}, "__main__", alter_sys=True)
+                 _val, {{}}, "__main__", alter_sys=True)
 
     if _args:
         sys.argv[:] = _args
         __file__ = _args[0]
         del _options, _args
-        with open(__file__, 'U') as __file__f:
+        with open(__file__{}) as __file__f:
             exec(compile(__file__f.read(), __file__, "exec"))
 
 if _interactive:
     del _interactive
     __import__("code").interact(banner="", local=globals())
-'''
+'''.format(universal_newline_option)
 
 runsetup_template = """
 import sys
@@ -1637,9 +1642,9 @@ __file__ = %(__file__)r
 os.chdir(%(setupdir)r)
 sys.argv[0] = %(setup)r
 
-with open(%(setup)r, 'U') as f:
+with open(%(setup)r{}) as f:
     exec(compile(f.read(), %(setup)r, 'exec'))
-"""
+""".format(universal_newline_option)
 
 
 class VersionConflict(zc.buildout.UserError):