[pycodestyle] Implement redundant-backslash (E502)

[augustelalande]

Summary

Implements the redundant-backslash rule (E502) from pycodestyle.

Test Plan

New fixture has been added

Part of #2402

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

ℹ️ ecosystem check detected linter changes. (+4 -0 violations, +0 -0 fixes in 1 projects; 42 projects unchanged)

bokeh/bokeh (+4 -0 violations, +0 -0 fixes)

ruff check --no-cache --exit-zero --ignore RUF9 --output-format concise --preview --select ALL

+ src/bokeh/io/webdriver.py:198:105: E502 [*] Redundant backslash
+ src/bokeh/io/webdriver.py:199:105: E502 [*] Redundant backslash
+ src/bokeh/server/contexts.py:304:102: E502 [*] Redundant backslash
+ src/bokeh/server/session.py:90:96: E502 [*] Redundant backslash

Changes by rule (1 rules affected)

code	total	+ violation	- violation	+ fix	- fix
E502	4	4	0	0	0

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[charliermarsh]

We have a struct called Indexer that already stores the locations of all continuations (i.e., the line positions following a \). I'm wondering if we could instead iterate over the token stream, identify parenthesized ranges, and then search for continuation_lines offsets within parenthesized ranges?

[augustelalande]

Done

[charliermarsh]

Thanks, this looks good to me.

[Avasam]

One more step towards #9057 🎉