Make benchmarks stable, the second attempt

[asvetlov]

Problematic benchmarks now ask not the single key, but three sibling keys.
These keys have different hashes, maybe it could help with amortizing benchmark time after hash collisions.

See #1201 for details

[codspeed-hq]

CodSpeed Performance Report

Merging #1206 will degrade performances by 79.79%

_{Comparing asvetlov:stable-bench (d6c8577) with master (06f1301)}

Summary

❌ 12 (👁 12) regressions
✅ 232 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
👁	test_cimultidict_getall_istr_hit[c]	102.4 µs	261.1 µs	-60.78%
👁	test_cimultidict_getall_istr_hit[py]	1.6 ms	8.1 ms	-79.79%
👁	test_cimultidict_getall_istr_miss[c]	28.5 µs	50.8 µs	-43.87%
👁	test_cimultidict_getall_istr_miss[py]	176.4 µs	537.5 µs	-67.18%
👁	test_multidict_getall_str_hit[ci-c]	112.7 µs	284.6 µs	-60.39%
👁	test_multidict_getall_str_hit[ci-py]	1.6 ms	8.1 ms	-79.7%
👁	test_multidict_getall_str_hit[cs-c]	99.1 µs	245.5 µs	-59.64%
👁	test_multidict_getall_str_hit[cs-py]	1.7 ms	8 ms	-78.5%
👁	test_multidict_getall_str_miss[ci-c]	39.1 µs	74.5 µs	-47.48%
👁	test_multidict_getall_str_miss[ci-py]	173.8 µs	576.8 µs	-69.87%
👁	test_multidict_getall_str_miss[cs-c]	25.6 µs	37.4 µs	-31.57%
👁	test_multidict_getall_str_miss[cs-py]	139 µs	439.8 µs	-68.38%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.32%. Comparing base (06f1301) to head (d6c8577).
⚠️ Report is 31 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1206      +/-   ##
==========================================
- Coverage   99.85%   98.32%   -1.53%     
==========================================
  Files          25       27       +2     
  Lines        3380     3874     +494     
  Branches      229      706     +477     
==========================================
+ Hits         3375     3809     +434     
- Misses          3       18      +15     
- Partials        2       47      +45     

Flag	Coverage Δ
CI-GHA	98.32% <100.00%> (-1.53%)	⬇️
MyPy	80.47% <100.00%> (?)
pytest	99.85% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.