fix: remove unused python-dotenv dependency

[micaelaw]

python-dotenv is declared as a runtime dependency in pyproject.toml but is never imported or used anywhere in the package. This removes it from pyproject.toml, requirements.txt, and regenerates poetry.lock.

Two motivations:

• My downstream project's Dependabot is hitting a version conflict since shipengine currently pins python-dotenv<0.16.0. Since the dependency isn't actually used, removing it entirely resolves the conflict.
• This fix: commit should give release-please a properly formatted conventional commit to parse, triggering a new release PR. The previous 2.1.0 bump never made it to PyPI because my merge commits didn't follow the Conventional Commits format, so release-please silently skipped them.

[micaelaw]

@semarj If you have an opportunity, I would appreciate help in getting this merged in. My main goal is to trigger release-please to open a new release, and then have that get pushed to PyPI. The version I made last week did not get pushed because my commits did not adhere to conventional commit structure, so I can't yet use the new get_rate_estimate endpoint I made when installing from PyPI. (I opened another PR to surface that conventional commit need to contributors.) Thank you for your help here!