Disable pyproject.toml dynamic version for release-please

[pndurette]

This PR,

• Disable pyproject.toml dynamic version for release-please (which it expects as of this writing)
• Set the version explicitly in pyproject.toml, hoping that release-please updates it + gtts/version.py 🤞🏻 (Edit: it DOES! 🎉 )

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (main@25d3c1c). Click here to learn what that means. The diff coverage is n/a.

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