Add typing hints to CaselessDict and Headers.

[wRAR]

Fixes #5989

[codecov]

Codecov Report

Merging #6097 (9942d3c) into master (c655679) will decrease coverage by 0.03%.
The diff coverage is 93.33%.

❗ Current head 9942d3c differs from pull request most recent head 26d2497. Consider uploading reports for the commit 26d2497 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6097      +/-   ##
==========================================
- Coverage   88.95%   88.92%   -0.03%     
==========================================
  Files         163      163              
  Lines       11571    11580       +9     
  Branches     1876     1878       +2     
==========================================
+ Hits        10293    10298       +5     
- Misses        969      971       +2     
- Partials      309      311       +2     

Files	Coverage Δ
scrapy/core/http2/stream.py	91.90% <100.00%> (ø)
scrapy/http/request/__init__.py	97.82% <100.00%> (ø)
scrapy/http/response/__init__.py	100.00% <100.00%> (ø)
scrapy/http/response/text.py	98.42% <100.00%> (ø)
scrapy/http/headers.py	95.94% <93.93%> (-2.59%)	⬇️
scrapy/utils/datatypes.py	98.14% <90.00%> (-1.86%)	⬇️