Use latest setuptools version supported for each Python version

[sfirrin]

What does this PR do?

Upgrades our pinned setuptools to the latest supported for each Python version.

setuptools dropped support for Python 2 in version 45 (link), so we need to specify an older version (44.1.1) for Python 2. This was done using the python_version environment marker documented here.

Motivation

#104

Testing Guidelines

Working as expected in the integration tests where we can see the older version is used for Python 2 and the newer version is used for Python 3.

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
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• This PR passes the integration tests (ask a Datadog member to run the tests)