remove cgi module usage, which causes deprecation warnings on Python 3.11

[wimglenn]

On Python 3.11, as soon as botocore.utils is imported there are deprecation warnings due to the import cgi, a module which is slated to be removed from stdlib. PEP 594 recommends replacing the cgi.parse_header with this email.message substitute.

[codecov-commenter]

Codecov Report

Base: 93.57% // Head: 93.58% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (5e14dd4) compared to base (72df8fa).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 5e14dd4 differs from pull request most recent head e891fa0. Consider uploading reports for the commit e891fa0 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2794      +/-   ##
===========================================
+ Coverage    93.57%   93.58%   +0.01%     
===========================================
  Files           63       63              
  Lines        13300    13302       +2     
===========================================
+ Hits         12445    12449       +4     
+ Misses         855      853       -2     

Impacted Files	Coverage Δ
botocore/utils.py	79.22% <100.00%> (+0.15%)	⬆️

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[wimglenn]

@nateprewitt Just saw you move CI to Python-3.11, could we get some eyes on this PR please?

[nateprewitt]

Hi @wimglenn, thanks for checking in. It's on our radar but we're currently looking at other options than email to replace this behavior. Once we have a conclusion on that, we'll revisit here with a decision. For the time being, there's no immediate risk of removal until October 2023 at the earliest with the release of 3.12.

Edit: I see it's slated for 3.13 in October 2025.

[wimglenn]

Interesting, is there some problem with the stdlib email recommendation from the PEP?
I'm not concerned about the removal in 3.13, but about the warning itself (some of us escalate warnings to errors in CI)

[nateprewitt]

Sure, and I can see how that's a problem if your builds are running with warnings on. I think we can merge this for now and if we end up with another solution, we can revisit that later. Thanks for the PR!