Add __reversed__ support to keys, values, and items views

[r266-tech]

Summary

Implements the __reversed__ method for _KeysView, _ValuesView, and _ItemsView, allowing reversed() iteration over multidict views.

Since Python 3.8, built-in dict views support reversed(). This PR brings multidict views to parity.

Closes #448.

Changes

• Added iter_entries_reverse() to _HtKeys for reverse entry traversal
• Added __reversed__ and _iter_reversed to _KeysView, _ValuesView, and _ItemsView
• All view types (including CIMultiDict and proxies) support reversed()

Example

from multidict import MultiDict

md = MultiDict([('a', 1), ('b', 2), ('c', 3)])
list(reversed(md.keys()))   # ['c', 'b', 'a']
list(reversed(md.values())) # [3, 2, 1]
list(reversed(md.items()))  # [('c', 3), ('b', 2), ('a', 1)]

[codspeed-hq]

Merging this PR will not alter performance

✅ 245 untouched benchmarks

---

_{Comparing r266-tech:add-reversed-to-views (bd1b5d6) with master (81fc6b9)}

[codecov]

Codecov Report

❌ Patch coverage is 30.43478% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.40%. Comparing base (81fc6b9) to head (bd1b5d6).
⚠️ Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
multidict/_multidict_py.py	30.43%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1304      +/-   ##
==========================================
- Coverage   99.85%   99.40%   -0.46%     
==========================================
  Files          26       26              
  Lines        3513     3536      +23     
  Branches      253      259       +6     
==========================================
+ Hits         3508     3515       +7     
- Misses          3       19      +16     
  Partials        2        2              

Flag	Coverage Δ
CI-GHA	99.40% <30.43%> (-0.46%)	⬇️
pytest	99.40% <30.43%> (-0.46%)	⬇️

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[Vizonex]

I would recommend you add a C Implementation and not just a Python One. This is because most users will end up with compiled binaries and multidict will default to the C Module if found.