Drop Python 3.7 support

[mshafer-NI]

Justification

• 3.7 has been EOL since June 27, 2023 (1 year and 11 months)
• We already stopped testing for it due to limitations of GitHub tooling
• Both our core tooling (flake8) and most of our dev tooling (notably, pytest) dropped it some time ago.

Implementation

• bump the 0.x version to denote a breaking change
• declare support for >=3.8,<4.0 (using the Poetry syntax)

Testing

PR checks

[github-actions]

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