[PR #6967/179312ea backport][3.9] Compute XOR table lazily (#7105)

**This is a backport of PR #6967 as merged into master
(179312e).**

This speeds up import time by a little bit:
```
In [2]: %timeit [bytes(a ^ b for a in range(256)) for b in range(256)]
8.41 ms ± 656 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
```
This is about 5% of aiohttp's import time

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

aiohttp/http_websocket.py

@@ -2,6 +2,7 @@
 
 import asyncio
 import collections
+import functools
 import json
 import random
 import re
@@ -121,7 +122,9 @@ class WSHandshakeError(Exception):
 
 
 # Used by _websocket_mask_python
-_XOR_TABLE: Final[List[bytes]] = [bytes(a ^ b for a in range(256)) for b in range(256)]
+@functools.lru_cache()
+def _xor_table() -> List[bytes]:
+ return [bytes(a ^ b for a in range(256)) for b in range(256)]
 
 
 def _websocket_mask_python(mask: bytes, data: bytearray) -> None:
@@ -141,6 +144,7 @@ def _websocket_mask_python(mask: bytes, data: bytearray) -> None:
  assert len(mask) == 4, mask
 
  if data:
+ _XOR_TABLE = _xor_table()
  a, b, c, d = (_XOR_TABLE[n] for n in mask)
  data[::4] = data[::4].translate(a)
  data[1::4] = data[1::4].translate(b)