Add support for setup.cfg-only projects

Many projects can get away with an empty `setup.py` and use *only* the
declarative `setup.cfg`. With the new PEP 517 backend, we can supply a
default empty `setup.py` if one is not provided.

setuptools/build_meta.py

@@ -26,6 +26,7 @@ bug reports or API stability):
 Again, this is not a formal definition! Just a "taste" of the module.
 """
 
+import io
 import os
 import sys
 import tokenize
@@ -95,6 +96,14 @@ def _file_with_extension(directory, extension):
     return file
 
 
+def _open_setup_script(setup_script):
+    if not os.path.exists(setup_script):
+        # Supply a default setup.py
+        return io.StringIO(u"from setuptools import setup; setup()")
+
+    return getattr(tokenize, 'open', open)(setup_script)
+
+
 class _BuildMetaBackend(object):
 
     def _fix_config(self, config_settings):
@@ -120,9 +129,10 @@ class _BuildMetaBackend(object):
         # Correctness comes first, then optimization later
         __file__ = setup_script
         __name__ = '__main__'
-        f = getattr(tokenize, 'open', open)(__file__)
-        code = f.read().replace('\\r\\n', '\\n')
-        f.close()
+
+        with _open_setup_script(__file__) as f:
+            code = f.read().replace(r'\r\n', r'\n')
+
         exec(compile(code, __file__, 'exec'), locals())
 
     def get_requires_for_build_wheel(self, config_settings=None):

setuptools/tests/test_build_meta.py

@@ -110,6 +110,21 @@ defns = [
                 print('hello')
             """),
     },
+    {
+        'setup.cfg': DALS("""
+        [metadata]
+        name = foo
+        version='0.0.0'
+
+        [options]
+        py_modules=hello
+        setup_requires=six
+        """),
+        'hello.py': DALS("""
+        def run():
+            print('hello')
+        """)
+    },
 ]
 
 
@@ -183,9 +198,13 @@ class TestBuildMetaBackend:
         # if the setup.py changes subsequent call of the build meta
         # should still succeed, given the
         # sdist_directory the frontend specifies is empty
-        with open(os.path.abspath("setup.py"), 'rt') as file_handler:
+        setup_loc = os.path.abspath("setup.py")
+        if not os.path.exists(setup_loc):
+            setup_loc = os.path.abspath("setup.cfg")
+
+        with open(setup_loc, 'rt') as file_handler:
             content = file_handler.read()
-        with open(os.path.abspath("setup.py"), 'wt') as file_handler:
+        with open(setup_loc, 'wt') as file_handler:
             file_handler.write(
                 content.replace("version='0.0.0'", "version='0.0.1'"))