diff --git a/main.py b/main.py
index d0ac950..2772127 100644
--- a/main.py
+++ b/main.py
@@ -1,6 +1,7 @@
 import re
 import subprocess
 import sys
+from typing import Union
 
 from src.errors import ClangError
 
@@ -23,10 +24,10 @@ def clang_format(file_path):
     return output
 
 
-def parse_clang_format_output(output) -> None | list[ClangError]:
+def parse_clang_format_output(output) -> Union[None, list[ClangError]]:
     error_pattern = (
         r"(?P<filename>.+):(?P<line_number>\d+):(?P<column_number>\d+):"
-        + r" warning: code should be clang-formatted \[(?P<warning_message>.+)\]"
+        r" warning: code should be clang-formatted \[(?P<warning_message>.+)\]"
     )
     error_matches = re.finditer(error_pattern, output)
 
diff --git a/pyproject.toml b/pyproject.toml
index 5d70e2b..f4196c0 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -1,8 +1,8 @@
 # https://black.readthedocs.io/en/stable/usage_and_configuration/the_basics.html#configuration-format
 [tool.black]
-target-version = ["py311"]
+target-version = ["py39"]
 
 # https://beta.ruff.rs/docs/settings/
 [tool.ruff]
-target-version = "py311"
+target-version = "py39"
 extend-select = ["I"] # enable isort extension