Python 3.11 (#9515)

## Problem

On Debian 12 (Bookworm), Python 3.11 is the latest available version.

## Summary of changes
- Update Python to 3.11 in build-tools
- Fix ruff check / format
- Fix mypy
- Use `StrEnum` instead of pair `str`, `Enum`
- Update docs

test_runner/performance/test_copy.py

@@ -2,7 +2,7 @@ from __future__ import annotations
 
 from contextlib import closing
 from io import BufferedReader, RawIOBase
-from typing import Optional, final
+from typing import final
 
 from fixtures.compare_fixtures import PgCompare
 from typing_extensions import override
@@ -13,7 +13,7 @@ class CopyTestData(RawIOBase):
     def __init__(self, rows: int):
         self.rows = rows
         self.rownum = 0
-        self.linebuf: Optional[bytes] = None
+        self.linebuf: bytes | None = None
         self.ptr = 0
 
     @override