Local implementation of find_pyproject_toml()

This ensures we use `find_project_root()` from Darkgraylib, avoiding
incompatibilities with different Black versions.

src/darker/black_diff.py

@@ -41,7 +41,6 @@
 from black import FileMode as Mode
 from black import (
     TargetVersion,
-    find_pyproject_toml,
     format_str,
     parse_pyproject_toml,
     re_compile_maybe_verbose,
@@ -53,6 +52,7 @@
 from black.files import gen_python_files
 from black.report import Report
 
+from darker.files import find_pyproject_toml
 from darkgraylib.config import ConfigurationError
 from darkgraylib.utils import TextDocument
 

src/darker/files.py

@@ -0,0 +1,27 @@
+from typing import Optional, Tuple
+
+from black import err, find_user_pyproject_toml
+
+from darkgraylib.files import find_project_root
+
+
+def find_pyproject_toml(
+    path_search_start: Tuple[str, ...], stdin_filename: Optional[str] = None
+) -> Optional[str]:
+    """Find the absolute filepath to a pyproject.toml if it exists"""
+    path_project_root, _ = find_project_root(path_search_start, stdin_filename)
+    path_pyproject_toml = path_project_root / "pyproject.toml"
+    if path_pyproject_toml.is_file():
+        return str(path_pyproject_toml)
+
+    try:
+        path_user_pyproject_toml = find_user_pyproject_toml()
+        return (
+            str(path_user_pyproject_toml)
+            if path_user_pyproject_toml.is_file()
+            else None
+        )
+    except (PermissionError, RuntimeError) as e:
+        # We do not have access to the user-level config directory, so ignore it.
+        err(f"Ignoring user configuration directory due to {e!r}")
+        return None