Add types for _compat.py

[kasium]

Add pyi files for cheroot._compat

❓ 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 #)

Part of #384

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

cheroot._compat is not typed

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

cheroot._compat is typed

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• 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 #491 (eeea95f) into master (22eda80) will decrease coverage by 0.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #491      +/-   ##
==========================================
- Coverage   80.56%   80.24%   -0.32%     
==========================================
  Files          28       28              
  Lines        4409     4409              
==========================================
- Hits         3552     3538      -14     
- Misses        857      871      +14     

[kasium]

@webknjaz can you please have a look? I guess the CI is a little bit flaky

[webknjaz]

xmlrpc.client.ProtocolError: <ProtocolError for pypi.python.org/pypi: 503 Service Unavailable>

This seems to be related to PyPI disabling XML-RPC.