Support loading version from a file

Author: jmbowman

docs/setuptools.txt

@@ -2421,7 +2421,7 @@ Metadata
 Key                             Aliases            Type
 ==============================  =================  =====
 name                                               str
-version                                            attr:, str
+version                                            attr:, file:, str
 url                             home-page          str
 download_url                    download-url       str
 project_urls                                       dict

setuptools/config.py

@@ -427,6 +427,11 @@ def _parse_version(self, value):
         :rtype: str
 
         """
+        version = self._parse_file(value)
+
+        if version != value:
+            return version.strip()
+
         version = self._parse_attr(value)
 
         if callable(version):

setuptools/tests/test_config.py

@@ -268,6 +268,18 @@ def test_version(self, tmpdir):
         with get_dist(tmpdir) as dist:
             assert dist.metadata.version == '2016.11.26'
 
+    def test_version_file(self, tmpdir):
+
+        _, config = fake_env(
+            tmpdir,
+            '[metadata]\n'
+            'version = file: fake_package/version.txt\n'
+        )
+        tmpdir.join('fake_package', 'version.txt').write('1.2.3\n')
+
+        with get_dist(tmpdir) as dist:
+            assert dist.metadata.version == '1.2.3'
+
     def test_unknown_meta_item(self, tmpdir):
 
         fake_env(