refactor!: Replace `httpx` with `impit` #598

Job	Run time
Type check / Type check (3.10)	33s
Type check / Type check (3.13)	30s
Lint check / Lint check (3.13)	18s
Lint check / Lint check (3.10)	16s
Lint check / Lint check (3.12)	11s
Type check / Type check (3.12)	27s
Type check / Type check (3.11)	28s
Unit tests / Unit tests (ubuntu-latest, 3.12)	30s
Unit tests / Unit tests (ubuntu-latest, 3.11)	22s
Unit tests / Unit tests (windows-latest, 3.11)	46s
Unit tests / Unit tests (windows-latest, 3.12)	44s
Lint check / Lint check (3.11)	10s
Unit tests / Unit tests (windows-latest, 3.10)	52s
Docs check / Docs check	1m 8s
Unit tests / Unit tests (ubuntu-latest, 3.10)	26s
Unit tests / Unit tests (ubuntu-latest, 3.13)	26s
Unit tests / Unit tests (windows-latest, 3.13)	1m 6s
Integration tests / Integration tests	11m 41s
Integration tests / Approve integration tests for PRs from forks	15h 9m 14s
Integration tests / Integration tests for PRs from forks	0s
	15h 30m 8s

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