DRAFT: Apply pyupgrade.

Just seeing how it looks like, we woudl of course need to pass options
in, so i'm thinking only once there is a config file; likely.

This is likely incorrect as the edited lines could likely be off once
black has been ran as it can rewrap stuff.

It also does use pyupgrade private internal API, and the author has
explicitly stated they they were not willing to have public API beyond
the CLI.

We should also likely run this _before_ black; maybe even before isort ?
not sure if it might remove imports... But at least it does not pass the
ast unchange part.

src/darker/__main__.py

@@ -125,14 +125,42 @@ def format_edited_parts(
                 # 10. A re-formatted Python file which produces an identical AST was
                 #     created successfully - write an updated file or print the diff
                 #     if there were any changes to the original
+                result_str = pyup(result_str, edited_lines, edited_linenums)
+
                 if result_str != worktree_content:
                     # `result_str` is just `chosen_lines` concatenated with newlines.
                     # We need both forms when showing diffs or modifying files.
                     # Pass them both on to avoid back-and-forth conversion.
-                    yield src, worktree_content, result_str, chosen_lines
+                    yield src, worktree_content, result_str, result_str.splitlines()
                 break
 
 
+def pyup(content, edited_lines, edited_linenums):
+
+    from pyupgrade import (
+        _fix_py2_compatible,
+        _fix_tokens,
+        _fix_percent_format,
+        _fix_py3_plus,
+        _fix_py36_plus,
+    )
+
+    min_version = (3, 6)
+    keep_mock = True
+    content = _fix_py2_compatible(content)
+    content = _fix_tokens(content, min_version)
+    content = _fix_percent_format(content)
+    content = _fix_py3_plus(content, min_version, keep_mock)
+    content = _fix_py36_plus(content)
+    content = content.splitlines()
+
+    pyup_opcodes = diff_and_get_opcodes(edited_lines, content)
+    pyupgrade_chunks = list(opcodes_to_chunks(pyup_opcodes, edited_lines, content))
+    pyup_chosen_lines: List[str] = list(choose_lines(pyupgrade_chunks, edited_linenums))
+    pyup_result_str = joinlines(pyup_chosen_lines)
+    return pyup_result_str
+
+
 def modify_file(path: Path, new_content: str) -> None:
     """Write new content to a file and inform the user by logging"""
     logger.info("Writing %s bytes into %s", len(new_content), path)