classifiers metadata name should be Classifier instead of `Classifi…

…ers`

https://packaging.python.org/specifications/core-metadata/#classifier-multiple-use

I've downloaded `Flask` and `requests` from PyPI, they all show
`Classifier` in wheel metadata

src/metadata.rs

@@ -361,7 +361,7 @@ impl Metadata21 {
         add_vec("Supported-Platform", &self.supported_platform);
         add_vec("Platform", &self.platform);
         add_vec("Supported-Platform", &self.supported_platform);
-        add_vec("Classifiers", &self.classifiers);
+        add_vec("Classifier", &self.classifiers);
         add_vec("Requires-Dist", &self.requires_dist);
         add_vec("Provides-Dist", &self.provides_dist);
         add_vec("Obsoletes-Dist", &self.obsoletes_dist);
@@ -542,7 +542,7 @@ mod test {
             Metadata-Version: 2.1
             Name: info-project
             Version: 0.1.0
-            Classifiers: Programming Language :: Python
+            Classifier: Programming Language :: Python
             Requires-Dist: flask~=1.1.0
             Requires-Dist: toml==0.10.0
             Summary: A test project
@@ -601,7 +601,7 @@ mod test {
             Metadata-Version: 2.1
             Name: info-project
             Version: 0.1.0
-            Classifiers: Programming Language :: Python
+            Classifier: Programming Language :: Python
             Requires-Dist: flask~=1.1.0
             Requires-Dist: toml==0.10.0
             Summary: A test project
@@ -649,7 +649,7 @@ mod test {
             Metadata-Version: 2.1
             Name: info
             Version: 0.1.0
-            Classifiers: Programming Language :: Python
+            Classifier: Programming Language :: Python
             Summary: A test project
             Home-Page: https://example.org
             Author: konstin <[email protected]>

src/upload.rs

@@ -77,7 +77,15 @@ pub fn upload(
         // Type system shenanigans
         .chain(metadata21.to_vec().into_iter())
         // All fields must be lower case and with underscores or they will be ignored by warehouse
-        .map(|(key, value)| (key.to_lowercase().replace("-", "_"), value))
+        .map(|(key, value)| {
+            let mut key = key.to_lowercase().replace("-", "_");
+            if key == "classifier" {
+                // PyPI upload api expects `classifiers` instead of `classifier`
+                // See https://github.com/pypa/warehouse/issues/3151#issuecomment-796965735
+                key = "classifiers".to_string();
+            }
+            (key, value)
+        })
         .collect();
 
     let mut form = Form::new();