Disallow files for license inputs

The ability to handle files was originally added and documented based on
a misunderstanding of what the `license` field should include. The field
should be the name of the license, not the full text.

It is likely that anyone actually using this was outputing malformed
PKG-INFO files, because most license files contain newlines.

See GH issue #1551

setup.cfg

@@ -26,4 +26,3 @@ universal = 1
 license_file = LICENSE
 
 [bumpversion:file:setup.py]
-

setuptools/config.py

@@ -246,6 +246,24 @@ class ConfigHandler:
         value = value.lower()
         return value in ('1', 'true', 'yes')
 
+    @classmethod
+    def _exclude_files_parser(cls, key):
+        """Returns a parser function to make sure field inputs
+        are not files.
+
+        Parses a value after getting the key so error messages are
+        more informative.
+
+        :param key:
+        :rtype: callable
+        """
+        def parser(value):
+            exclude_directive = 'file:'
+            if value.startswith(exclude_directive):
+                raise ValueError('Only strings are accepted for the {0} field, files are not accepted'.format(key))
+            return value
+        return parser
+
     @classmethod
     def _parse_file(cls, value):
         """Represents value as a string, allowing including text
@@ -255,7 +273,6 @@ class ConfigHandler:
         directory with setup.py.
 
         Examples:
-            file: LICENSE
             file: README.rst, CHANGELOG.md, src/file.txt
 
         :param str value:
@@ -449,6 +466,7 @@ class ConfigMetadataHandler(ConfigHandler):
         parse_list = self._parse_list
         parse_file = self._parse_file
         parse_dict = self._parse_dict
+        exclude_files_parser = self._exclude_files_parser
 
         return {
             'platforms': parse_list,
@@ -460,7 +478,7 @@ class ConfigMetadataHandler(ConfigHandler):
                 DeprecationWarning),
             'obsoletes': parse_list,
             'classifiers': self._get_parser_compound(parse_file, parse_list),
-            'license': parse_file,
+            'license': exclude_files_parser('license'),
             'description': parse_file,
             'long_description': parse_file,
             'version': self._parse_version,

setuptools/tests/test_egg_info.py

@@ -148,6 +148,26 @@ class TestEggInfo:
         ]
         assert sorted(actual) == expected
 
+    def test_license_is_a_string(self, tmpdir_cwd, env):
+        setup_config = DALS("""
+            [metadata]
+            name=foo
+            version=0.0.1
+            license=file:MIT
+            """)
+
+        setup_script = DALS("""
+            from setuptools import setup
+
+            setup()
+            """)
+
+        build_files({'setup.py': setup_script,
+                     'setup.cfg': setup_config})
+
+        with pytest.raises(ValueError):
+            self._run_egg_info_command(tmpdir_cwd, env)
+
     def test_rebuilt(self, tmpdir_cwd, env):
         """Ensure timestamps are updated when the command is re-run."""
         self._create_project()
@@ -598,7 +618,10 @@ class TestEggInfo:
             env=environ,
         )
         if code:
-            raise AssertionError(data)
+            if 'ValueError' in data:
+                raise ValueError(data)
+            else:
+                raise AssertionError(data)
         if output:
             assert output in data