[PR #6205/f8b07dd6 backport][3.9] Set the minimum required setuptools to v46.4.0

[patchback]

This is a backport of PR #6205 as merged into master (f8b07dd).

In order to avoid a recursive dependency issue when parsing the aiohttp version during install setuptools needs:
pypa/setuptools@c457e68

This implies we need setuptools 46.4.0 or newer.

This should probably also be back-ported as this setuptools version requirement was introduced in 3.8.

What do these changes do?

Adds setuptools >= 46.4.0 requirement to pyproject.toml and readme.

Are there changes in behavior for the user?

Should ensure setuptools version is correct instead of cryptic build dependency errors.

Related issue number

N/A

Checklist

• [N/A] I think the code is well written
• [N/A] Unit tests for the changes exist
• Documentation reflects the changes
• [N/A] If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov]

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@@          Coverage Diff           @@
##             3.9    #6226   +/-   ##
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  Coverage       ?   93.30%           
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  Files          ?      103           
  Lines          ?    30360           
  Branches       ?     2729           
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  Hits           ?    28326           
  Misses         ?     1857           
  Partials       ?      177           

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