dev(narugo): use httpx instead of requests

[narugo1992]

• use httpx instead of requests
• but some of the sites are using 3rd-party libraries:
  • [ ] pixiv (using pixivpy3)
  • [ ] anime_pictures (using cloudscraper)
  • [ ] wallhaven (using cloudscraper)
• [x] unittest migration

[codecov[bot]]

Codecov Report

Attention: 20 lines in your changes are missing coverage. Please review.

Comparison is base (a4d18f3) 65.94% compared to head (05066ac) 58.82%.

Files	Patch %	Lines
waifuc/utils/session.py	57.89%	8 Missing :warning:
waifuc/utils/download.py	77.41%	7 Missing :warning:
waifuc/source/web.py	50.00%	3 Missing :warning:
waifuc/source/anime_pictures.py	50.00%	1 Missing :warning:
waifuc/source/danbooru.py	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #26 +/- ## ========================================== - Coverage 65.94% 58.82% -7.12% ========================================== Files 44 45 +1 Lines 2170 2181 +11 ========================================== - Hits 1431 1283 -148 - Misses 739 898 +159 ``` | [Flag](https://app.codecov.io/gh/deepghs/waifuc/pull/26/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepghs) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/deepghs/waifuc/pull/26/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepghs) | `58.82% <72.97%> (-7.12%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=deepghs#carryforward-flags-in-the-pull-request-comment) to find out more.

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