Add .codspeed folder to .gitignore

[asvetlov]

The folder is created by tests running; it should never be added to the git tree.

[codspeed-hq]

CodSpeed Performance Report

Merging #1188 will degrade performances by 18.93%

_{Comparing asvetlov:ignore-codspeed (0b3fea7) with master (70fa9c5)}

Summary

⚡ 3 improvements
❌ 3 regressions
✅ 238 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	test_cimultidict_getall_istr_hit[pure-python-module]	119.7 µs	142.3 µs	-15.89%
⚡	test_cimultidict_getall_istr_miss[pure-python-module]	42 µs	34.8 µs	+20.57%
❌	test_multidict_getall_str_hit[case-insensitive-pure-python-module]	118.7 µs	140.2 µs	-15.29%
❌	test_multidict_getall_str_hit[case-sensitive-pure-python-module]	98 µs	120.9 µs	-18.93%
⚡	test_multidict_getall_str_miss[case-insensitive-pure-python-module]	40.8 µs	33.7 µs	+21%
⚡	test_multidict_getall_str_miss[case-sensitive-pure-python-module]	35.2 µs	30.1 µs	+16.76%