fix(package): Ignore setup.cfg attr: and file: (#3054)

The setup.cfg version key allows "attr:" and "file:" directives to load
the actual value from elsewhere [1]. Treat these as if no version was
found, since the text of the directive isn't the version number we're
after.

[1]: https://setuptools.readthedocs.io/en/latest/userguide/declarative_config.html#specifying-values

src/modules/package.rs

@@ -84,7 +84,11 @@ fn get_setup_cfg_version(context: &Context, config: &PackageConfig) -> Option<St
     let ini = Ini::load_from_str(&file_contents).ok()?;
     let raw_version = ini.get_from(Some("metadata"), "version")?;
 
-    format_version(raw_version, config.version_format)
+    if raw_version.starts_with("attr:") || raw_version.starts_with("file:") {
+        None
+    } else {
+        format_version(raw_version, config.version_format)
+    }
 }
 
 fn get_gradle_version(context: &Context, config: &PackageConfig) -> Option<String> {
@@ -613,6 +617,34 @@ license = "MIT"
         project_dir.close()
     }
 
+    #[test]
+    fn test_extract_setup_cfg_version_attr() -> io::Result<()> {
+        let config_name = "setup.cfg";
+        let config_content = String::from(
+            "[metadata]
+            version = attr: mymod.__version__",
+        );
+
+        let project_dir = create_project_dir()?;
+        fill_config(&project_dir, config_name, Some(&config_content))?;
+        expect_output(&project_dir, None, None);
+        project_dir.close()
+    }
+
+    #[test]
+    fn test_extract_setup_cfg_version_file() -> io::Result<()> {
+        let config_name = "setup.cfg";
+        let config_content = String::from(
+            "[metadata]
+            version = file: version.txt",
+        );
+
+        let project_dir = create_project_dir()?;
+        fill_config(&project_dir, config_name, Some(&config_content))?;
+        expect_output(&project_dir, None, None);
+        project_dir.close()
+    }
+
     #[test]
     fn test_extract_gradle_version_single_quote() -> io::Result<()> {
         let config_name = "build.gradle";