Relax setuptools build dependency specification

[jakob-keller]

Description of Change

Relax setuptools build dependency specification to deal with constraints in external build contexts: closes #1193

Assumptions

I have performed local tests: The resulting distribution packages are nearly identical with only minor variations in metadata, primarily due to formatting, and no functional differences.

Checklist for All Submissions

• I have added change info to CHANGES.rst
• If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details):
  • Detailed description of issue
  • Alternative methods considered (if any)
  • How issue is being resolved
  • How issue can be reproduced
• If this is providing a new feature (can be provided via link to issue with these details):
  • Detailed description of new feature
  • Why needed
  • Alternatives methods considered (if any)

Checklist when updating botocore and/or aiohttp versions

• I have read and followed CONTRIBUTING.rst
• I have updated test_patches.py where/if appropriate (also check if no changes necessary)
• I have ensured that the awscli/boto3 versions match the updated botocore version

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.99%. Comparing base (35ce0ef) to head (24a44d7).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1194   +/-   ##
=======================================
  Coverage   86.99%   86.99%           
=======================================
  Files          64       64           
  Lines        5983     5983           
=======================================
  Hits         5205     5205           
  Misses        778      778           

Flag	Coverage Δ
unittests	86.99% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.