Do not use pydantic for settings validation

[Cadair]

@SolarDrew found an issue with pydantic where you couldn't pass the SSL Context into aiohttp_session_kwargs, I couldn't figure it out so I gave up

[codecov]

Codecov Report

Merging #98 (2e18740) into main (f3dac92) will increase coverage by 0.22%.
The diff coverage is 100.00%.

❗ Current head 2e18740 differs from pull request most recent head f8c2370. Consider uploading reports for the commit f8c2370 to get more accurate results

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
+ Coverage   93.33%   93.55%   +0.22%     
==========================================
  Files           5        5              
  Lines         540      559      +19     
==========================================
+ Hits          504      523      +19     
  Misses         36       36              

Impacted Files	Coverage Δ
parfive/config.py	99.07% <100.00%> (+0.19%)	⬆️
parfive/downloader.py	93.26% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f3dac92...f8c2370. Read the comment docs.