Drop support for Python 3.5 and 2.7

[jaraco]

• Add a py2-drop banner to the docs
• Remove declared support for Python < 3.6
• Remove compatibility code from wsgi
• Remove compatibility code from cheroot
• Remove compatibility code from cli
• Remove compatibility code from connections
• Remove compatibility code from errors
• Remove compatibility code from makefile
• Remove compatibility code from test__compat
• Remove compatibility code from server
• Remove compatibility code from testing
• Remove compatibility code from threadpool
• Remove compatibility code from ssl package
• Remove compatibility code from test package
• Remove Python 3.5 compatibility from _compat module
• Add a changelog entry for v9.0.0

❓ What kind of change does this PR introduce?

• 🐞 bug fix
• 🐣 feature
• 📋 docs update
• 📋 tests/coverage improvement
• 📋 refactoring
• 💥 other

📋 What is the related issue number (starting with #)

Resolves #252

❓ What is the current behavior? (You can also link to an open issue here)

❓ What is the new behavior (if this is a feature change)?

📋 Other information:

📋 Contribution checklist:

(If you're a first-timer, check out
this guide on making great pull requests)

• I wrote descriptive pull request text above
• I think the code is well written
• I wrote good commit messages
• I have squashed related commits together after
  the changes have been approved
• Unit tests for the changes exist
• Integration tests for the changes exist (if applicable)
• I used the same coding conventions as the rest of the project
• The new code doesn't generate linter offenses
• Documentation reflects the changes
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences

---

This change is 

[codecov]

Codecov Report

Merging #510 (1396980) into main (f0f9764) will increase coverage by 1.45%.
The diff coverage is 82.69%.

❗ Current head 1396980 differs from pull request most recent head 06f2107. Consider uploading reports for the commit 06f2107 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #510      +/-   ##
==========================================
+ Coverage   80.80%   82.26%   +1.45%     
==========================================
  Files          27       28       +1     
  Lines        4413     3991     -422     
==========================================
- Hits         3566     3283     -283     
+ Misses        847      708     -139     

[lgtm-com]

This pull request fixes 4 alerts when merging 1396980 into b7be66d - view on LGTM.com

fixed alerts:

• 3 for Unused import
• 1 for Unused local variable

[lgtm-com]

This pull request fixes 4 alerts when merging ea92dc0 into f0f9764 - view on LGTM.com

fixed alerts:

• 3 for Unused import
• 1 for Unused local variable

---

Heads-up: LGTM.com's PR analysis will be disabled on the 5th of December, and LGTM.com will be shut down ⏻ completely on the 16th of December 2022. Please enable GitHub code scanning, which uses the same CodeQL engine ⚙️ that powers LGTM.com. For more information, please check out our post on the GitHub blog.

[lgtm-com]

This pull request fixes 4 alerts when merging 06f2107 into f0f9764 - view on LGTM.com

fixed alerts:

• 3 for Unused import
• 1 for Unused local variable

---

Heads-up: LGTM.com's PR analysis will be disabled on the 5th of December, and LGTM.com will be shut down ⏻ completely on the 16th of December 2022. Please enable GitHub code scanning, which uses the same CodeQL engine ⚙️ that powers LGTM.com. For more information, please check out our post on the GitHub blog.