Improve type safety according to pyright

Pyright found many issues that mypy doesn't seem to want to catch or
mypy isn't configured to catch.

Signed-off-by: Tristan Partin <tristan@neon.tech>

test_runner/performance/test_copy.py

@@ -2,11 +2,13 @@ from __future__ import annotations
 
 from contextlib import closing
 from io import BufferedReader, RawIOBase
-from typing import Optional
+from typing import Optional, final
 
 from fixtures.compare_fixtures import PgCompare
+from typing_extensions import override
 
 
+@final
 class CopyTestData(RawIOBase):
     def __init__(self, rows: int):
         self.rows = rows
@@ -14,6 +16,7 @@ class CopyTestData(RawIOBase):
         self.linebuf: Optional[bytes] = None
         self.ptr = 0
 
+    @override
     def readable(self):
         return True