Add trailing comma for single-element import-from groups (#6583)

## Summary

Unlike other statements, Black always adds a trailing comma if an
import-from statement breaks with a single import member. I believe this
is for compatibility with isort -- see
09f5ee3a19,
https://github.com/psf/black/issues/127, or
66648c528a/src/black/linegen.py (L1452)
for the current version.

## Test Plan

`cargo test`, notice that a big chunk of the compatibility suite is
removed.

Before:

| project      | similarity index |
|--------------|------------------|
| cpython      | 0.75472          |
| django       | 0.99804          |
| transformers | 0.99618          |
| twine        | 0.99876          |
| typeshed     | 0.74233          |
| warehouse    | 0.99601          |
| zulip        | 0.99727          |

After:

| project      | similarity index |
|--------------|------------------|
| cpython      | 0.75472          |
| django       | 0.99804          |
| transformers | 0.99618          |
| twine        | 0.99876          |
| typeshed     | 0.74260          |
| warehouse    | 0.99601          |
| zulip        | 0.99727          |

crates/ruff_python_formatter/src/builders.rs

@@ -92,11 +92,23 @@ impl Entries {
     }
 }
 
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Default)]
+pub(crate) enum TrailingComma {
+    /// Add a trailing comma if the group breaks and there's more than one element (or if the last
+    /// element has a trailing comma and the magical trailing comma option is enabled).
+    #[default]
+    MoreThanOne,
+    /// Add a trailing comma if the group breaks (or if the last element has a trailing comma and
+    /// the magical trailing comma option is enabled).
+    OneOrMore,
+}
+
 pub(crate) struct JoinCommaSeparatedBuilder<'fmt, 'ast, 'buf> {
     result: FormatResult<()>,
     fmt: &'fmt mut PyFormatter<'ast, 'buf>,
     entries: Entries,
     sequence_end: TextSize,
+    trailing_comma: TrailingComma,
 }
 
 impl<'fmt, 'ast, 'buf> JoinCommaSeparatedBuilder<'fmt, 'ast, 'buf> {
@@ -106,9 +118,19 @@ impl<'fmt, 'ast, 'buf> JoinCommaSeparatedBuilder<'fmt, 'ast, 'buf> {
             result: Ok(()),
             entries: Entries::None,
             sequence_end,
+            trailing_comma: TrailingComma::default(),
         }
     }
 
+    /// Set the trailing comma behavior for the builder. Trailing commas will only be inserted if
+    /// the group breaks, and will _always_ be inserted if the last element has a trailing comma
+    /// (and the magical trailing comma option is enabled). However, this setting dictates whether
+    /// trailing commas are inserted for single element groups.
+    pub(crate) fn with_trailing_comma(mut self, trailing_comma: TrailingComma) -> Self {
+        self.trailing_comma = trailing_comma;
+        self
+    }
+
     pub(crate) fn entry<T>(
         &mut self,
         node: &T,
@@ -194,8 +216,11 @@ impl<'fmt, 'ast, 'buf> JoinCommaSeparatedBuilder<'fmt, 'ast, 'buf> {
                 };
 
                 // If there is a single entry, only keep the magic trailing comma, don't add it if
-                // it wasn't there. If there is more than one entry, always add it.
-                if magic_trailing_comma || self.entries.is_more_than_one() {
+                // it wasn't there -- unless the trailing comma behavior is set to one-or-more.
+                if magic_trailing_comma
+                    || self.trailing_comma == TrailingComma::OneOrMore
+                    || self.entries.is_more_than_one()
+                {
                     if_group_breaks(&text(",")).fmt(self.fmt)?;
                 }
 

crates/ruff_python_formatter/src/statement/stmt_import_from.rs

@@ -3,7 +3,7 @@ use ruff_formatter::{write, Buffer, Format, FormatResult};
 use ruff_python_ast::node::AstNode;
 use ruff_python_ast::{Ranged, StmtImportFrom};
 
-use crate::builders::{parenthesize_if_expands, PyFormatterExtensions};
+use crate::builders::{parenthesize_if_expands, PyFormatterExtensions, TrailingComma};
 use crate::expression::parentheses::parenthesized;
 use crate::{AsFormat, FormatNodeRule, PyFormatter};
 
@@ -45,6 +45,7 @@ impl FormatNodeRule<StmtImportFrom> for FormatStmtImportFrom {
 
         let names = format_with(|f| {
             f.join_comma_separated(item.end())
+                .with_trailing_comma(TrailingComma::OneOrMore)
                 .entries(names.iter().map(|name| (name, name.format())))
                 .finish()
         });