Remove support for passing a bytes key to HTTPHeaderDict

[jdufresne]

No description provided.

[jdufresne]

Motivation for this change started in thread: #2395 (comment)

[codecov]

Codecov Report

Merging #2397 (1f2b22a) into main (52e9830) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2397      +/-   ##
==========================================
- Coverage   99.95%   99.95%   -0.01%     
==========================================
  Files          25       25              
  Lines        2472     2468       -4     
==========================================
- Hits         2471     2467       -4     
  Misses          1        1              

Impacted Files	Coverage Δ
src/urllib3/_collections.py	100.00% <ø> (ø)

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[sethmlarson]

Even though this is a "major" release we're trying to minimize breaking changes while opening up potential for future improvements. Supporting byte keys is both fortunately (because imo it's the correct way to do HTTP headers) and unfortunately (strings are easy to work with, users prefer them, it's what our API has historically been) widely used. So we're kinda soft-disallowing them in v2.0 via type hints but still secretly accepting them.

[jdufresne]

Thanks for clarifying. I have updated #2395 with this in mind and will close this PR.