`difflib.HtmlDiff` has quadratic line pairing for lopsided replacements

[felix314159]

difflib._mdiff() buffers unmatched lines in lists and consumes them using
pop(0). For lopsided replacement blocks, each removal shifts the remaining
items, making line pairing quadratic.

Reproducer:

import difflib

a = ["a\n"] * 64_000
b = ["b\n"]

list(difflib._mdiff(a, b))

Using matched release builds from CPython main:

Case	Current	With deque.popleft()
_mdiff(), 64,000-to-1	264 ms	61.9 ms
HtmlDiff.make_table(), 64,000-to-1	398 ms	206 ms
_mdiff(), 128,000-to-1	999 ms	126 ms

The speedup increases with input size, reaching 7.94x at 128,000 lines. Patched
runtime scales approximately linearly.

Using collections.deque for the two pairing queues preserves FIFO behavior
while making removal constant-time. Focused tests, the complete test_difflib
suite, and randomized differential testing confirm identical output.

Tested on CPython main at ed1aa1f324f9 on Linux.

Linked PRs

• gh-156505: Speed up difflib.HtmlDiff for lopsided replacements #156506