[3.15] gh-153030: Fix quadratic complexity in incremental parsing in HTMLParser ...

[3.15] gh-153030: Fix quadratic complexity in incremental parsing in HTMLParser (GH-153031) (GH-153038)

When an unterminated construct (e.g. a tag or comment) spanned many
feed() calls, rescanning the growing buffer and concatenating new data
onto it were both quadratic.  New data is now accumulated in a list and
only joined and parsed once enough has piled up.
(cherry picked from commit bcf98ddbc40ec9b3ee87da0124a5660b19b7e606)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>